diff --git a/.idea/codeStyleSettings.xml b/.idea/codeStyleSettings.xml
index 01c5a434f..ca933b417 100644
--- a/.idea/codeStyleSettings.xml
+++ b/.idea/codeStyleSettings.xml
@@ -29,7 +29,6 @@
           </indentOptions>
         </codeStyleSettings>
         <codeStyleSettings language="JAVA">
-          <option name="KEEP_LINE_BREAKS" value="false" />
           <option name="KEEP_CONTROL_STATEMENT_IN_ONE_LINE" value="false" />
           <option name="KEEP_BLANK_LINES_IN_DECLARATIONS" value="1" />
           <option name="KEEP_BLANK_LINES_IN_CODE" value="1" />
diff --git a/.travis.yml b/.travis.yml
index a99de7177..7f73bbd0b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -40,7 +40,7 @@ script:
   - pushd tests/src/test/js
   - export DISPLAY=:99.0
   - sh -e /etc/init.d/xvfb start
-  - sleep 3
+  - sleep 10
   - firefox index.html &
   - FIREFOX_PID=$!
   - node start.js $BASE_PATH/tests/target/js-tests
diff --git a/core/src/main/java/org/teavm/ast/decompilation/Decompiler.java b/core/src/main/java/org/teavm/ast/decompilation/Decompiler.java
index 1a1cb5261..acae70ed9 100644
--- a/core/src/main/java/org/teavm/ast/decompilation/Decompiler.java
+++ b/core/src/main/java/org/teavm/ast/decompilation/Decompiler.java
@@ -91,14 +91,16 @@ public class Decompiler {
     private List<TryCatchBookmark> tryCatchBookmarks = new ArrayList<>();
     private Deque<Block> stack;
     private Program program;
+    private boolean friendlyToDebugger;
 
     public Decompiler(ClassHolderSource classSource, ClassLoader classLoader, Set<MethodReference> asyncMethods,
-            Set<MethodReference> asyncFamilyMethods) {
+            Set<MethodReference> asyncFamilyMethods, boolean friendlyToDebugger) {
         this.classSource = classSource;
         this.classLoader = classLoader;
         this.asyncMethods = asyncMethods;
         splitMethods.addAll(asyncMethods);
         splitMethods.addAll(asyncFamilyMethods);
+        this.friendlyToDebugger = friendlyToDebugger;
     }
 
     public MethodNodeCache getRegularMethodCache() {
@@ -275,7 +277,7 @@ public class Decompiler {
         }
 
         Optimizer optimizer = new Optimizer();
-        optimizer.optimize(methodNode, method.getProgram());
+        optimizer.optimize(methodNode, method.getProgram(), friendlyToDebugger);
         methodNode.getModifiers().addAll(method.getModifiers());
 
         return methodNode;
@@ -339,7 +341,7 @@ public class Decompiler {
         }
 
         Optimizer optimizer = new Optimizer();
-        optimizer.optimize(node, splitter);
+        optimizer.optimize(node, splitter, friendlyToDebugger);
         node.getModifiers().addAll(method.getModifiers());
 
         return node;
diff --git a/core/src/main/java/org/teavm/ast/optimization/Optimizer.java b/core/src/main/java/org/teavm/ast/optimization/Optimizer.java
index ee0b8afd1..a977fa287 100644
--- a/core/src/main/java/org/teavm/ast/optimization/Optimizer.java
+++ b/core/src/main/java/org/teavm/ast/optimization/Optimizer.java
@@ -18,7 +18,9 @@ package org.teavm.ast.optimization;
 import java.util.BitSet;
 import org.teavm.ast.AsyncMethodNode;
 import org.teavm.ast.AsyncMethodPart;
+import org.teavm.ast.MethodNode;
 import org.teavm.ast.RegularMethodNode;
+import org.teavm.ast.VariableNode;
 import org.teavm.common.Graph;
 import org.teavm.model.BasicBlock;
 import org.teavm.model.Instruction;
@@ -31,13 +33,14 @@ import org.teavm.model.util.ProgramUtils;
 import org.teavm.model.util.UsageExtractor;
 
 public class Optimizer {
-    public void optimize(RegularMethodNode method, Program program) {
+    public void optimize(RegularMethodNode method, Program program, boolean friendlyToDebugger) {
         ReadWriteStatsBuilder stats = new ReadWriteStatsBuilder(method.getVariables().size());
         stats.analyze(program);
         boolean[] preservedVars = new boolean[stats.writes.length];
         BreakEliminator breakEliminator = new BreakEliminator();
         breakEliminator.eliminate(method.getBody());
-        OptimizingVisitor optimizer = new OptimizingVisitor(preservedVars, stats.writes, stats.reads);
+        OptimizingVisitor optimizer = new OptimizingVisitor(preservedVars, stats.writes, stats.reads,
+                friendlyToDebugger);
         method.getBody().acceptVisitor(optimizer);
         method.setBody(optimizer.resultStmt);
         int paramCount = method.getReference().parameterCount();
@@ -55,7 +58,7 @@ public class Optimizer {
         }
     }
 
-    public void optimize(AsyncMethodNode method, AsyncProgramSplitter splitter) {
+    public void optimize(AsyncMethodNode method, AsyncProgramSplitter splitter, boolean friendlyToDebugger) {
         LivenessAnalyzer liveness = new LivenessAnalyzer();
         liveness.analyze(splitter.getOriginalProgram());
 
@@ -70,7 +73,8 @@ public class Optimizer {
             BreakEliminator breakEliminator = new BreakEliminator();
             breakEliminator.eliminate(part.getStatement());
             findEscapingLiveVars(liveness, cfg, splitter, i, preservedVars);
-            OptimizingVisitor optimizer = new OptimizingVisitor(preservedVars, stats.writes, stats.reads);
+            OptimizingVisitor optimizer = new OptimizingVisitor(preservedVars, stats.writes, stats.reads,
+                    friendlyToDebugger);
             part.getStatement().acceptVisitor(optimizer);
             part.setStatement(optimizer.resultStmt);
         }
@@ -93,6 +97,14 @@ public class Optimizer {
         }
     }
 
+    private void preserveDebuggableVars(boolean[] variablesToPreserve, MethodNode methodNode) {
+        for (VariableNode varNode : methodNode.getVariables()) {
+            if (varNode.getName() != null) {
+                variablesToPreserve[varNode.getIndex()] = true;
+            }
+        }
+    }
+
     private void findEscapingLiveVars(LivenessAnalyzer liveness, Graph cfg, AsyncProgramSplitter splitter,
             int partIndex, boolean[] output) {
         Program originalProgram = splitter.getOriginalProgram();
diff --git a/core/src/main/java/org/teavm/ast/optimization/OptimizingVisitor.java b/core/src/main/java/org/teavm/ast/optimization/OptimizingVisitor.java
index 8dc0deb64..7b589bda8 100644
--- a/core/src/main/java/org/teavm/ast/optimization/OptimizingVisitor.java
+++ b/core/src/main/java/org/teavm/ast/optimization/OptimizingVisitor.java
@@ -15,8 +15,11 @@
  */
 package org.teavm.ast.optimization;
 
+import java.util.ArrayDeque;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Deque;
+import java.util.LinkedList;
 import java.util.List;
 import java.util.Objects;
 import java.util.stream.Collectors;
@@ -58,6 +61,7 @@ import org.teavm.ast.UnaryOperation;
 import org.teavm.ast.UnwrapArrayExpr;
 import org.teavm.ast.VariableExpr;
 import org.teavm.ast.WhileStatement;
+import org.teavm.model.TextLocation;
 
 class OptimizingVisitor implements StatementVisitor, ExprVisitor {
     private Expr resultExpr;
@@ -66,11 +70,17 @@ class OptimizingVisitor implements StatementVisitor, ExprVisitor {
     private final int[] writeFrequencies;
     private final int[] readFrequencies;
     private List<Statement> resultSequence;
+    private boolean friendlyToDebugger;
+    private TextLocation currentLocation;
+    private Deque<TextLocation> locationStack = new LinkedList<>();
+    private Deque<TextLocation> notNullLocationStack = new ArrayDeque<>();
 
-    OptimizingVisitor(boolean[] preservedVars, int[] writeFrequencies, int[] readFrequencies) {
+    OptimizingVisitor(boolean[] preservedVars, int[] writeFrequencies, int[] readFrequencies,
+            boolean friendlyToDebugger) {
         this.preservedVars = preservedVars;
         this.writeFrequencies = writeFrequencies;
         this.readFrequencies = readFrequencies;
+        this.friendlyToDebugger = friendlyToDebugger;
     }
 
     private static boolean isZero(Expr expr) {
@@ -81,74 +91,100 @@ class OptimizingVisitor implements StatementVisitor, ExprVisitor {
         return expr instanceof BinaryExpr && ((BinaryExpr) expr).getOperation() == BinaryOperation.COMPARE;
     }
 
+    private void pushLocation(TextLocation location) {
+        locationStack.push(location);
+        if (location != null) {
+            if (currentLocation != null) {
+                notNullLocationStack.push(currentLocation);
+            }
+            currentLocation = location;
+        }
+    }
+
+    private void popLocation() {
+        if (locationStack.pop() != null) {
+            currentLocation = notNullLocationStack.pollFirst();
+        }
+    }
+
     @Override
     public void visit(BinaryExpr expr) {
-        switch (expr.getOperation()) {
-            case AND:
-            case OR:
-                resultExpr = expr;
-                return;
-            default:
-                break;
-        }
-        expr.getSecondOperand().acceptVisitor(this);
-        Expr b = resultExpr;
-        if (b instanceof ConstantExpr && expr.getOperation() == BinaryOperation.SUBTRACT) {
-            if (tryMakePositive((ConstantExpr) b)) {
-                expr.setOperation(BinaryOperation.ADD);
-            }
-        }
-        expr.getFirstOperand().acceptVisitor(this);
-        Expr a = resultExpr;
-        Expr p = a;
-        Expr q = b;
-        boolean invert = false;
-        if (isZero(p)) {
-            Expr tmp = p;
-            p = q;
-            q = tmp;
-            invert = true;
-        }
-        if (isComparison(p) && isZero(q)) {
+        pushLocation(expr.getLocation());
+        try {
             switch (expr.getOperation()) {
-                case EQUALS:
-                case NOT_EQUALS:
-                case LESS:
-                case LESS_OR_EQUALS:
-                case GREATER:
-                case GREATER_OR_EQUALS: {
-                    BinaryExpr comparison = (BinaryExpr) p;
-                    Expr result = BinaryExpr.binary(expr.getOperation(), comparison.getType(),
-                            comparison.getFirstOperand(), comparison.getSecondOperand());
-                    result.setLocation(comparison.getLocation());
-                    if (invert) {
-                        result = ExprOptimizer.invert(result);
-                    }
-                    resultExpr = result;
+                case AND:
+                case OR:
+                    resultExpr = expr;
                     return;
-                }
                 default:
                     break;
             }
+            expr.getSecondOperand().acceptVisitor(this);
+            Expr b = resultExpr;
+            if (b instanceof ConstantExpr && expr.getOperation() == BinaryOperation.SUBTRACT) {
+                if (tryMakePositive((ConstantExpr) b)) {
+                    expr.setOperation(BinaryOperation.ADD);
+                }
+            }
+            expr.getFirstOperand().acceptVisitor(this);
+            Expr a = resultExpr;
+            Expr p = a;
+            Expr q = b;
+            boolean invert = false;
+            if (isZero(p)) {
+                Expr tmp = p;
+                p = q;
+                q = tmp;
+                invert = true;
+            }
+            if (isComparison(p) && isZero(q)) {
+                switch (expr.getOperation()) {
+                    case EQUALS:
+                    case NOT_EQUALS:
+                    case LESS:
+                    case LESS_OR_EQUALS:
+                    case GREATER:
+                    case GREATER_OR_EQUALS: {
+                        BinaryExpr comparison = (BinaryExpr) p;
+                        Expr result = BinaryExpr.binary(expr.getOperation(), comparison.getType(),
+                                comparison.getFirstOperand(), comparison.getSecondOperand());
+                        result.setLocation(comparison.getLocation());
+                        if (invert) {
+                            result = ExprOptimizer.invert(result);
+                        }
+                        resultExpr = result;
+                        return;
+                    }
+                    default:
+                        break;
+                }
+            }
+            expr.setFirstOperand(a);
+            expr.setSecondOperand(b);
+            resultExpr = expr;
+        } finally {
+            popLocation();
         }
-        expr.setFirstOperand(a);
-        expr.setSecondOperand(b);
-        resultExpr = expr;
     }
 
     @Override
     public void visit(UnaryExpr expr) {
-        expr.getOperand().acceptVisitor(this);
-        Expr operand = resultExpr;
-        if (expr.getOperation() == UnaryOperation.NEGATE && operand instanceof ConstantExpr) {
-            ConstantExpr constantExpr = (ConstantExpr) operand;
-            if (tryMakePositive(constantExpr)) {
-                resultExpr = expr;
-                return;
+        pushLocation(expr.getLocation());
+        try {
+            expr.getOperand().acceptVisitor(this);
+            Expr operand = resultExpr;
+            if (expr.getOperation() == UnaryOperation.NEGATE && operand instanceof ConstantExpr) {
+                ConstantExpr constantExpr = (ConstantExpr) operand;
+                if (tryMakePositive(constantExpr)) {
+                    resultExpr = expr;
+                    return;
+                }
             }
+            expr.setOperand(operand);
+            resultExpr = expr;
+        } finally {
+             popLocation();
         }
-        expr.setOperand(operand);
-        resultExpr = expr;
     }
 
     private boolean tryMakePositive(ConstantExpr constantExpr) {
@@ -177,16 +213,21 @@ class OptimizingVisitor implements StatementVisitor, ExprVisitor {
 
     @Override
     public void visit(ConditionalExpr expr) {
-        expr.getCondition().acceptVisitor(this);
-        Expr cond = resultExpr;
-        expr.getConsequent().acceptVisitor(this);
-        Expr consequent = resultExpr;
-        expr.getAlternative().acceptVisitor(this);
-        Expr alternative = resultExpr;
-        expr.setCondition(cond);
-        expr.setConsequent(consequent);
-        expr.setAlternative(alternative);
-        resultExpr = expr;
+        pushLocation(expr.getLocation());
+        try {
+            expr.getCondition().acceptVisitor(this);
+            Expr cond = resultExpr;
+            expr.getConsequent().acceptVisitor(this);
+            Expr consequent = resultExpr;
+            expr.getAlternative().acceptVisitor(this);
+            Expr alternative = resultExpr;
+            expr.setCondition(cond);
+            expr.setConsequent(consequent);
+            expr.setAlternative(alternative);
+            resultExpr = expr;
+        } finally {
+            popLocation();
+        }
     }
 
     @Override
@@ -196,70 +237,92 @@ class OptimizingVisitor implements StatementVisitor, ExprVisitor {
 
     @Override
     public void visit(VariableExpr expr) {
-        int index = expr.getIndex();
-        resultExpr = expr;
-        if (writeFrequencies[index] != 1) {
-            return;
-        }
-        if (readFrequencies[index] != 1 || preservedVars[index]) {
-            return;
-        }
-        if (resultSequence.isEmpty()) {
-            return;
-        }
-        Statement last = resultSequence.get(resultSequence.size() - 1);
-        if (!(last instanceof AssignmentStatement)) {
-            return;
-        }
-        AssignmentStatement assignment = (AssignmentStatement) last;
-        if (assignment.isAsync()) {
-            return;
-        }
-        if (!(assignment.getLeftValue() instanceof VariableExpr)) {
-            return;
-        }
-        VariableExpr var = (VariableExpr) assignment.getLeftValue();
-        if (var.getLocation() != null && assignment.getLocation() != null
-                && !assignment.getLocation().equals(var.getLocation())) {
-            return;
-        }
-        if (var.getIndex() == index) {
-            resultSequence.remove(resultSequence.size() - 1);
-            assignment.getRightValue().setLocation(assignment.getLocation());
-            assignment.getRightValue().acceptVisitor(this);
+        pushLocation(expr.getLocation());
+        try {
+            int index = expr.getIndex();
+            resultExpr = expr;
+            if (writeFrequencies[index] != 1) {
+                return;
+            }
+            if (readFrequencies[index] != 1 || preservedVars[index]) {
+                return;
+            }
+            if (resultSequence.isEmpty()) {
+                return;
+            }
+            Statement last = resultSequence.get(resultSequence.size() - 1);
+            if (!(last instanceof AssignmentStatement)) {
+                return;
+            }
+            AssignmentStatement assignment = (AssignmentStatement) last;
+            if (assignment.isAsync()) {
+                return;
+            }
+            if (!(assignment.getLeftValue() instanceof VariableExpr)) {
+                return;
+            }
+            VariableExpr var = (VariableExpr) assignment.getLeftValue();
+            if (friendlyToDebugger) {
+                if (currentLocation != null && assignment.getLocation() != null
+                        && !assignment.getLocation().equals(currentLocation)) {
+                    return;
+                }
+            }
+            if (var.getIndex() == index) {
+                resultSequence.remove(resultSequence.size() - 1);
+                assignment.getRightValue().setLocation(assignment.getLocation());
+                assignment.getRightValue().acceptVisitor(this);
+            }
+        } finally {
+            popLocation();
         }
     }
 
     @Override
     public void visit(SubscriptExpr expr) {
-        expr.getIndex().acceptVisitor(this);
-        Expr index = resultExpr;
-        expr.getArray().acceptVisitor(this);
-        Expr array = resultExpr;
-        expr.setArray(array);
-        expr.setIndex(index);
-        resultExpr = expr;
+        pushLocation(expr.getLocation());
+        try {
+            expr.getIndex().acceptVisitor(this);
+            Expr index = resultExpr;
+            expr.getArray().acceptVisitor(this);
+            Expr array = resultExpr;
+            expr.setArray(array);
+            expr.setIndex(index);
+            resultExpr = expr;
+        } finally {
+            popLocation();
+        }
     }
 
     @Override
     public void visit(UnwrapArrayExpr expr) {
-        expr.getArray().acceptVisitor(this);
-        Expr arrayExpr = resultExpr;
-        expr.setArray(arrayExpr);
-        resultExpr = expr;
+        pushLocation(expr.getLocation());
+        try {
+            expr.getArray().acceptVisitor(this);
+            Expr arrayExpr = resultExpr;
+            expr.setArray(arrayExpr);
+            resultExpr = expr;
+        } finally {
+            popLocation();
+        }
     }
 
     @Override
     public void visit(InvocationExpr expr) {
-        Expr[] args = new Expr[expr.getArguments().size()];
-        for (int i = expr.getArguments().size() - 1; i >= 0; --i) {
-            expr.getArguments().get(i).acceptVisitor(this);
-            args[i] = resultExpr;
+        pushLocation(expr.getLocation());
+        try {
+            Expr[] args = new Expr[expr.getArguments().size()];
+            for (int i = expr.getArguments().size() - 1; i >= 0; --i) {
+                expr.getArguments().get(i).acceptVisitor(this);
+                args[i] = resultExpr;
+            }
+            for (int i = 0; i < args.length; ++i) {
+                expr.getArguments().set(i, args[i]);
+            }
+            resultExpr = expr;
+        } finally {
+            popLocation();
         }
-        for (int i = 0; i < args.length; ++i) {
-            expr.getArguments().set(i, args[i]);
-        }
-        resultExpr = expr;
     }
 
     private boolean tryApplyConstructor(InvocationExpr expr) {
@@ -303,12 +366,17 @@ class OptimizingVisitor implements StatementVisitor, ExprVisitor {
 
     @Override
     public void visit(QualificationExpr expr) {
-        if (expr.getQualified() != null) {
-            expr.getQualified().acceptVisitor(this);
-            Expr qualified = resultExpr;
-            expr.setQualified(qualified);
+        pushLocation(expr.getLocation());
+        try {
+            if (expr.getQualified() != null) {
+                expr.getQualified().acceptVisitor(this);
+                Expr qualified = resultExpr;
+                expr.setQualified(qualified);
+            }
+            resultExpr = expr;
+        } finally {
+            popLocation();
         }
-        resultExpr = expr;
     }
 
     @Override
@@ -318,64 +386,94 @@ class OptimizingVisitor implements StatementVisitor, ExprVisitor {
 
     @Override
     public void visit(NewArrayExpr expr) {
-        expr.getLength().acceptVisitor(this);
-        Expr length = resultExpr;
-        expr.setLength(length);
-        resultExpr = expr;
+        pushLocation(expr.getLocation());
+        try {
+            expr.getLength().acceptVisitor(this);
+            Expr length = resultExpr;
+            expr.setLength(length);
+            resultExpr = expr;
+        } finally {
+            popLocation();
+        }
     }
 
     @Override
     public void visit(NewMultiArrayExpr expr) {
-        for (int i = 0; i < expr.getDimensions().size(); ++i) {
-            Expr dimension = expr.getDimensions().get(i);
-            dimension.acceptVisitor(this);
-            expr.getDimensions().set(i, resultExpr);
+        pushLocation(expr.getLocation());
+        try {
+            for (int i = 0; i < expr.getDimensions().size(); ++i) {
+                Expr dimension = expr.getDimensions().get(i);
+                dimension.acceptVisitor(this);
+                expr.getDimensions().set(i, resultExpr);
+            }
+            resultExpr = expr;
+        } finally {
+            popLocation();
         }
-        resultExpr = expr;
     }
 
     @Override
     public void visit(InstanceOfExpr expr) {
-        expr.getExpr().acceptVisitor(this);
-        expr.setExpr(resultExpr);
-        resultExpr = expr;
+        pushLocation(expr.getLocation());
+        try {
+            expr.getExpr().acceptVisitor(this);
+            expr.setExpr(resultExpr);
+            resultExpr = expr;
+        } finally {
+            popLocation();
+        }
     }
 
     @Override
     public void visit(CastExpr expr) {
-        expr.getValue().acceptVisitor(this);
-        expr.setValue(resultExpr);
-        resultExpr = expr;
+        pushLocation(expr.getLocation());
+        try {
+            expr.getValue().acceptVisitor(this);
+            expr.setValue(resultExpr);
+            resultExpr = expr;
+        } finally {
+            popLocation();
+        }
     }
 
     @Override
     public void visit(PrimitiveCastExpr expr) {
-        expr.getValue().acceptVisitor(this);
-        expr.setValue(resultExpr);
-        resultExpr = expr;
+        pushLocation(expr.getLocation());
+        try {
+            expr.getValue().acceptVisitor(this);
+            expr.setValue(resultExpr);
+            resultExpr = expr;
+        } finally {
+            popLocation();
+        }
     }
 
     @Override
     public void visit(AssignmentStatement statement) {
-        if (statement.getLeftValue() == null) {
-            statement.getRightValue().acceptVisitor(this);
-            if (resultExpr instanceof InvocationExpr && tryApplyConstructor((InvocationExpr) resultExpr)) {
-                resultStmt = new SequentialStatement();
+        pushLocation(statement.getLocation());
+        try {
+            if (statement.getLeftValue() == null) {
+                statement.getRightValue().acceptVisitor(this);
+                if (resultExpr instanceof InvocationExpr && tryApplyConstructor((InvocationExpr) resultExpr)) {
+                    resultStmt = new SequentialStatement();
+                } else {
+                    statement.setRightValue(resultExpr);
+                    resultStmt = statement;
+                }
             } else {
-                statement.setRightValue(resultExpr);
+                statement.getRightValue().acceptVisitor(this);
+                Expr right = resultExpr;
+                Expr left = statement.getLeftValue();
+                if (!(statement.getLeftValue() instanceof VariableExpr)) {
+                    statement.getLeftValue().acceptVisitor(this);
+                    left = resultExpr;
+                }
+                statement.setLeftValue(left);
+                statement.setRightValue(right);
                 resultStmt = statement;
             }
-        } else {
-            statement.getRightValue().acceptVisitor(this);
-            Expr right = resultExpr;
-            Expr left = statement.getLeftValue();
-            if (!(statement.getLeftValue() instanceof VariableExpr)) {
-                statement.getLeftValue().acceptVisitor(this);
-                left = resultExpr;
-            }
-            statement.setLeftValue(left);
-            statement.setRightValue(right);
-            resultStmt = statement;
+        } finally {
+            popLocation();
         }
     }
 
@@ -724,23 +822,38 @@ class OptimizingVisitor implements StatementVisitor, ExprVisitor {
 
     @Override
     public void visit(ReturnStatement statement) {
-        if (statement.getResult() != null) {
-            statement.getResult().acceptVisitor(this);
-            statement.setResult(resultExpr);
+        pushLocation(statement.getLocation());
+        try {
+            if (statement.getResult() != null) {
+                statement.getResult().acceptVisitor(this);
+                statement.setResult(resultExpr);
+            }
+            resultStmt = statement;
+        } finally {
+            popLocation();
         }
-        resultStmt = statement;
     }
 
     @Override
     public void visit(ThrowStatement statement) {
-        statement.getException().acceptVisitor(this);
-        statement.setException(resultExpr);
-        resultStmt = statement;
+        pushLocation(statement.getLocation());
+        try {
+            statement.getException().acceptVisitor(this);
+            statement.setException(resultExpr);
+            resultStmt = statement;
+        } finally {
+            popLocation();
+        }
     }
 
     @Override
     public void visit(InitClassStatement statement) {
-        resultStmt = statement;
+        pushLocation(statement.getLocation());
+        try {
+            resultStmt = statement;
+        } finally {
+            popLocation();
+        }
     }
 
     @Override
@@ -761,15 +874,25 @@ class OptimizingVisitor implements StatementVisitor, ExprVisitor {
 
     @Override
     public void visit(MonitorEnterStatement statement) {
-        statement.getObjectRef().acceptVisitor(this);
-        statement.setObjectRef(resultExpr);
-        resultStmt = statement;
+        pushLocation(statement.getLocation());
+        try {
+            statement.getObjectRef().acceptVisitor(this);
+            statement.setObjectRef(resultExpr);
+            resultStmt = statement;
+        } finally {
+            popLocation();
+        }
     }
 
     @Override
     public void visit(MonitorExitStatement statement) {
-        statement.getObjectRef().acceptVisitor(this);
-        statement.setObjectRef(resultExpr);
-        resultStmt = statement;
+        pushLocation(statement.getLocation());
+        try {
+            statement.getObjectRef().acceptVisitor(this);
+            statement.setObjectRef(resultExpr);
+            resultStmt = statement;
+        } finally {
+            popLocation();
+        }
     }
 }
diff --git a/core/src/main/java/org/teavm/backend/javascript/JavaScriptTarget.java b/core/src/main/java/org/teavm/backend/javascript/JavaScriptTarget.java
index 49407c0c2..a552683af 100644
--- a/core/src/main/java/org/teavm/backend/javascript/JavaScriptTarget.java
+++ b/core/src/main/java/org/teavm/backend/javascript/JavaScriptTarget.java
@@ -294,7 +294,8 @@ public class JavaScriptTarget implements TeaVMTarget, TeaVMJavaScriptHost {
         asyncMethods.addAll(asyncFinder.getAsyncMethods());
         asyncFamilyMethods.addAll(asyncFinder.getAsyncFamilyMethods());
 
-        Decompiler decompiler = new Decompiler(classes, controller.getClassLoader(), asyncMethods, asyncFamilyMethods);
+        Decompiler decompiler = new Decompiler(classes, controller.getClassLoader(), asyncMethods, asyncFamilyMethods,
+                controller.isFriendlyToDebugger());
         decompiler.setRegularMethodCache(controller.isIncremental() ? astCache : null);
 
         for (Map.Entry<MethodReference, Generator> entry : methodGenerators.entrySet()) {
diff --git a/core/src/main/java/org/teavm/backend/javascript/rendering/StatementRenderer.java b/core/src/main/java/org/teavm/backend/javascript/rendering/StatementRenderer.java
index 310c41588..f823c1677 100644
--- a/core/src/main/java/org/teavm/backend/javascript/rendering/StatementRenderer.java
+++ b/core/src/main/java/org/teavm/backend/javascript/rendering/StatementRenderer.java
@@ -926,7 +926,7 @@ public class StatementRenderer implements ExprVisitor, StatementVisitor {
                     break;
             }
             if (expr.getLocation() != null) {
-                pushLocation(expr.getLocation());
+                popLocation();
             }
         } catch (IOException e) {
             throw new RenderingException("IO error occured", e);
diff --git a/core/src/main/java/org/teavm/backend/wasm/WasmTarget.java b/core/src/main/java/org/teavm/backend/wasm/WasmTarget.java
index 83361c23c..20cda3548 100644
--- a/core/src/main/java/org/teavm/backend/wasm/WasmTarget.java
+++ b/core/src/main/java/org/teavm/backend/wasm/WasmTarget.java
@@ -291,7 +291,7 @@ public class WasmTarget implements TeaVMTarget, TeaVMWasmHost {
                 classes, vtableProvider, tagRegistry, binaryWriter);
 
         Decompiler decompiler = new Decompiler(classes, controller.getClassLoader(), new HashSet<>(),
-                new HashSet<>());
+                new HashSet<>(), false);
         WasmStringPool stringPool = classGenerator.getStringPool();
         WasmGenerationContext context = new WasmGenerationContext(classes, module, controller.getDiagnostics(),
                 vtableProvider, tagRegistry, stringPool);
diff --git a/core/src/main/java/org/teavm/debugging/Debugger.java b/core/src/main/java/org/teavm/debugging/Debugger.java
index 83da9c00e..4df730210 100644
--- a/core/src/main/java/org/teavm/debugging/Debugger.java
+++ b/core/src/main/java/org/teavm/debugging/Debugger.java
@@ -34,8 +34,8 @@ public class Debugger {
     private ConcurrentMap<String, ConcurrentMap<DebugInformation, Object>> debugInformationFileMap =
             new ConcurrentHashMap<>();
     private ConcurrentMap<DebugInformation, String> scriptMap = new ConcurrentHashMap<>();
-    final ConcurrentMap<JavaScriptBreakpoint, Breakpoint> breakpointMap = new ConcurrentHashMap<>();
-    ConcurrentMap<Breakpoint, Object> breakpoints = new ConcurrentHashMap<>();
+    private final ConcurrentMap<JavaScriptBreakpoint, Breakpoint> breakpointMap = new ConcurrentHashMap<>();
+    private final ConcurrentMap<Breakpoint, Object> breakpoints = new ConcurrentHashMap<>();
     private volatile CallFrame[] callStack;
 
     public Debugger(JavaScriptDebugger javaScriptDebugger, DebugInformationProvider debugInformationProvider) {
@@ -126,12 +126,12 @@ public class Debugger {
         javaScriptDebugger.resume();
     }
 
-    private static class CallSiteSuccessorFinder implements DebuggerCallSiteVisitor {
+    static class CallSiteSuccessorFinder implements DebuggerCallSiteVisitor {
         private DebugInformation debugInfo;
         private String script;
         Set<JavaScriptLocation> locations;
 
-        public CallSiteSuccessorFinder(DebugInformation debugInfo, String script, Set<JavaScriptLocation> locations) {
+        CallSiteSuccessorFinder(DebugInformation debugInfo, String script, Set<JavaScriptLocation> locations) {
             this.debugInfo = debugInfo;
             this.script = script;
             this.locations = locations;
@@ -185,7 +185,7 @@ public class Debugger {
 
     private List<DebugInformation> debugInformationBySource(String sourceFile) {
         Map<DebugInformation, Object> list = debugInformationFileMap.get(sourceFile);
-        return list != null ? new ArrayList<>(list.keySet()) : Collections.<DebugInformation>emptyList();
+        return list != null ? new ArrayList<>(list.keySet()) : Collections.emptyList();
     }
 
     public void continueToLocation(SourceLocation location) {
@@ -218,6 +218,10 @@ public class Debugger {
         return createBreakpoint(new SourceLocation(file, line));
     }
 
+    public Collection<? extends String> getSourceFiles() {
+        return debugInformationFileMap.keySet();
+    }
+
     public Breakpoint createBreakpoint(SourceLocation location) {
         synchronized (breakpointMap) {
             Breakpoint breakpoint = new Breakpoint(this, location);
@@ -232,7 +236,7 @@ public class Debugger {
         return new HashSet<>(breakpoints.keySet());
     }
 
-    void updateInternalBreakpoints(Breakpoint breakpoint) {
+    private void updateInternalBreakpoints(Breakpoint breakpoint) {
         if (breakpoint.isDestroyed()) {
             return;
         }
@@ -259,7 +263,7 @@ public class Debugger {
         return listeners.keySet().toArray(new DebuggerListener[0]);
     }
 
-    void updateBreakpointStatus(Breakpoint breakpoint, boolean fireEvent) {
+    private void updateBreakpointStatus(Breakpoint breakpoint, boolean fireEvent) {
         boolean valid = false;
         for (JavaScriptBreakpoint jsBreakpoint : breakpoint.jsBreakpoints) {
             if (jsBreakpoint.isValid()) {
diff --git a/core/src/main/java/org/teavm/debugging/VariableMap.java b/core/src/main/java/org/teavm/debugging/VariableMap.java
index 28d34da30..cbb511d14 100644
--- a/core/src/main/java/org/teavm/debugging/VariableMap.java
+++ b/core/src/main/java/org/teavm/debugging/VariableMap.java
@@ -63,6 +63,9 @@ class VariableMap extends AbstractMap<String, Variable> {
             String[] names = debugger.mapVariable(entry.getKey(), location);
             Value value = new Value(debugger, jsVar.getValue());
             for (String name : names) {
+                if (name == null) {
+                    name = "js:" + jsVar.getName();
+                }
                 vars.put(name, new Variable(name, value));
             }
         }
diff --git a/core/src/main/java/org/teavm/debugging/information/DebugInformation.java b/core/src/main/java/org/teavm/debugging/information/DebugInformation.java
index aeffe26e5..1fc27673c 100644
--- a/core/src/main/java/org/teavm/debugging/information/DebugInformation.java
+++ b/core/src/main/java/org/teavm/debugging/information/DebugInformation.java
@@ -216,7 +216,7 @@ public class DebugInformation {
         int[] valueIndexes = mapping.get(keyIndex).getArray(0);
         String[] result = new String[valueIndexes.length];
         for (int i = 0; i < result.length; ++i) {
-            result[i] = variableNames[valueIndexes[i]];
+            result[i] = valueIndexes[i] >= 0 ? variableNames[valueIndexes[i]] : null;
         }
         return result;
     }
diff --git a/core/src/main/java/org/teavm/debugging/information/DebugInformationBuilder.java b/core/src/main/java/org/teavm/debugging/information/DebugInformationBuilder.java
index f009b4710..3e3a7fbff 100644
--- a/core/src/main/java/org/teavm/debugging/information/DebugInformationBuilder.java
+++ b/core/src/main/java/org/teavm/debugging/information/DebugInformationBuilder.java
@@ -70,7 +70,7 @@ public class DebugInformationBuilder implements DebugInformationEmitter {
         }
     }
 
-    private  RecordArrayBuilder.Record add(RecordArrayBuilder builder) {
+    private RecordArrayBuilder.Record add(RecordArrayBuilder builder) {
         if (builder.size() > 1) {
             RecordArrayBuilder.Record lastRecord = builder.get(builder.size() - 1);
             if (lastRecord.get(0) == locationProvider.getLine() && lastRecord.get(1) == locationProvider.getColumn()) {
@@ -132,11 +132,8 @@ public class DebugInformationBuilder implements DebugInformationEmitter {
         }
         Arrays.sort(sourceIndexes);
         int generatedIndex = variableNames.index(generatedName);
-        RecordArrayBuilder mapping = variableMappings.get(generatedIndex);
-        if (mapping == null) {
-            mapping = new RecordArrayBuilder(2, 1);
-            variableMappings.put(generatedIndex, mapping);
-        }
+        RecordArrayBuilder mapping = variableMappings.computeIfAbsent(generatedIndex,
+                k -> new RecordArrayBuilder(2, 1));
 
         RecordArrayBuilder.Record record = add(mapping);
         RecordArrayBuilder.SubArray array = record.getArray(0);
diff --git a/core/src/main/java/org/teavm/vm/TeaVM.java b/core/src/main/java/org/teavm/vm/TeaVM.java
index 72330e8b8..18866252b 100644
--- a/core/src/main/java/org/teavm/vm/TeaVM.java
+++ b/core/src/main/java/org/teavm/vm/TeaVM.java
@@ -661,5 +661,10 @@ public class TeaVM implements TeaVMHost, ServiceRepository {
         public Map<String, String> getExportedClasses() {
             return readonlyExportedClasses;
         }
+
+        @Override
+        public boolean isFriendlyToDebugger() {
+            return optimizationLevel == TeaVMOptimizationLevel.SIMPLE;
+        }
     };
 }
diff --git a/core/src/main/java/org/teavm/vm/TeaVMTargetController.java b/core/src/main/java/org/teavm/vm/TeaVMTargetController.java
index 28ff0125c..6e3ac4ee6 100644
--- a/core/src/main/java/org/teavm/vm/TeaVMTargetController.java
+++ b/core/src/main/java/org/teavm/vm/TeaVMTargetController.java
@@ -39,6 +39,8 @@ public interface TeaVMTargetController {
 
     boolean isIncremental();
 
+    boolean isFriendlyToDebugger();
+
     Map<String, TeaVMEntryPoint> getEntryPoints();
 
     Map<String, String> getExportedClasses();
diff --git a/tools/chrome-rdp/src/main/java/org/teavm/chromerdp/ChromeRDPDebugger.java b/tools/chrome-rdp/src/main/java/org/teavm/chromerdp/ChromeRDPDebugger.java
index fbaee548f..a3e8123b5 100644
--- a/tools/chrome-rdp/src/main/java/org/teavm/chromerdp/ChromeRDPDebugger.java
+++ b/tools/chrome-rdp/src/main/java/org/teavm/chromerdp/ChromeRDPDebugger.java
@@ -24,6 +24,8 @@ import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
@@ -119,7 +121,12 @@ public class ChromeRDPDebugger implements JavaScriptDebugger, ChromeRDPExchangeC
                         }
                     }
                     CompletableFuture<Object> future = futures.remove(response.getId());
-                    responseHandlers.remove(response.getId()).received(response.getResult(), future);
+                    try {
+                        responseHandlers.remove(response.getId()).received(response.getResult(), future);
+                    } catch (RuntimeException e) {
+                        logger.warn("Error processing message ${}", response.getId(), e);
+                        future.completeExceptionally(e);
+                    }
                 } else {
                     Message message = mapper.reader(Message.class).readValue(messageText);
                     if (message.getMethod() == null) {
@@ -386,7 +393,7 @@ public class ChromeRDPDebugger implements JavaScriptDebugger, ChromeRDPExchangeC
 
         try {
             return read(sync);
-        } catch (InterruptedException e) {
+        } catch (InterruptedException | TimeoutException e) {
             return Collections.emptyList();
         }
     }
@@ -421,6 +428,8 @@ public class ChromeRDPDebugger implements JavaScriptDebugger, ChromeRDPExchangeC
             return result.isEmpty() ? null : result;
         } catch (InterruptedException e) {
             return null;
+        } catch (TimeoutException e) {
+            return "<timed out>";
         }
     }
 
@@ -454,6 +463,8 @@ public class ChromeRDPDebugger implements JavaScriptDebugger, ChromeRDPExchangeC
             return result.repr;
         } catch (InterruptedException e) {
             return null;
+        } catch (TimeoutException e) {
+            return "<timed out>";
         }
     }
 
@@ -559,9 +570,9 @@ public class ChromeRDPDebugger implements JavaScriptDebugger, ChromeRDPExchangeC
         void received(JsonNode node, CompletableFuture<T> out) throws IOException;
     }
 
-    private static <T> T read(Future<T> future) throws InterruptedException {
+    private static <T> T read(Future<T> future) throws InterruptedException, TimeoutException {
         try {
-            return future.get();
+            return future.get(1500, TimeUnit.MILLISECONDS);
         } catch (ExecutionException e) {
             Throwable cause = e.getCause();
             if (cause instanceof RuntimeException) {
diff --git a/tools/chrome-rdp/src/main/java/org/teavm/chromerdp/ChromeRDPRunner.java b/tools/chrome-rdp/src/main/java/org/teavm/chromerdp/ChromeRDPRunner.java
index 4a00eb463..de15c0ac4 100644
--- a/tools/chrome-rdp/src/main/java/org/teavm/chromerdp/ChromeRDPRunner.java
+++ b/tools/chrome-rdp/src/main/java/org/teavm/chromerdp/ChromeRDPRunner.java
@@ -51,6 +51,11 @@ public final class ChromeRDPRunner {
         new Thread(server::start).start();
         debugger = new Debugger(jsDebugger, new URLDebugInformationProvider(""));
         debugger.addListener(listener);
+
+        Thread.setDefaultUncaughtExceptionHandler((t, e) -> {
+            System.err.println("Uncaught exception in thread " + t);
+            e.printStackTrace();
+        });
     }
 
     private DebuggerListener listener = new DebuggerListener() {
@@ -221,12 +226,55 @@ public final class ChromeRDPRunner {
             System.out.println("Expected 2 arguments");
             return;
         }
-        Breakpoint bp = debugger.createBreakpoint(args[1], Integer.parseInt(args[2]));
+
+        String[] fileNames = resolveFileName(args[1]);
+        if (fileNames.length == 0) {
+            System.out.println("Unknown file: " + args[1]);
+            return;
+        } else if (fileNames.length > 1) {
+            System.out.println("Ambiguous file name: " + args[1] + ". Possible names are: "
+                    + Arrays.toString(fileNames));
+            return;
+        }
+
+        Breakpoint bp = debugger.createBreakpoint(fileNames[0], Integer.parseInt(args[2]));
         int id = breakpointIdGen++;
         breakpointIds.put(bp, id);
         System.out.println("Breakpoint #" + id + " was set at " + bp.getLocation());
     };
 
+    private String[] resolveFileName(String fileName) {
+        if (debugger.getSourceFiles().contains(fileName)) {
+            return new String[] { fileName };
+        }
+
+        String[] result = debugger.getSourceFiles().stream()
+                .filter(f -> f.endsWith(fileName) && isPrecededByPathSeparator(f, fileName))
+                .toArray(String[]::new);
+        if (result.length == 1) {
+            return result;
+        }
+
+        return debugger.getSourceFiles().stream()
+                .filter(f -> {
+                    int index = f.lastIndexOf('.');
+                    if (index <= 0) {
+                        return false;
+                    }
+                    String nameWithoutExt = f.substring(0, index);
+                    return nameWithoutExt.endsWith(fileName)  && isPrecededByPathSeparator(nameWithoutExt, fileName);
+                })
+                .toArray(String[]::new);
+    }
+
+    private static boolean isPrecededByPathSeparator(String actualName, String specifiedName) {
+        if (actualName.length() < specifiedName.length() + 1) {
+            return false;
+        }
+        char c = actualName.charAt(actualName.length() - specifiedName.length() - 1);
+        return c == '/' || c == '\\';
+    }
+
     private Command backtraceCommand = args -> {
         CallFrame[] callStack = debugger.getCallStack();
         for (int i = 0; i < callStack.length; ++i) {