Made some changes based on Alexey's comments of previous changes. Still getting same error on build.

teavm-core/src/main/java/org/teavm/cache/ProgramIO.java

@@ -597,12 +597,24 @@ public class ProgramIO {
 
         @Override
         public void visit(MonitorEnterInstruction insn) {
-            
+            try {
+                output.writeByte(39);
+                output.writeShort(insn.getObjectRef().getIndex());
+                
+            } catch (IOException e) {
+                throw new IOExceptionWrapper(e);
+            }
         }
 
         @Override
         public void visit(MonitorExitInstruction insn) {
-            
+            try {
+                output.writeByte(40);
+                output.writeShort(insn.getObjectRef().getIndex());
+                
+            } catch (IOException e) {
+                throw new IOExceptionWrapper(e);
+            }
         }
     }
 
@@ -908,6 +920,16 @@ public class ProgramIO {
                 insn.setValue(program.variableAt(input.readShort()));
                 return insn;
             }
+            case 39: {
+                MonitorEnterInstruction insn = new MonitorEnterInstruction();
+                insn.setObjectRef(program.variableAt(input.readShort()));
+                return insn;
+            }
+            case 40: {
+                MonitorExitInstruction insn = new MonitorExitInstruction();
+                insn.setObjectRef(program.variableAt(input.readShort()));
+                return insn;
+            }
             default:
                 throw new RuntimeException("Unknown instruction type: " + insnType);
         }

teavm-core/src/main/java/org/teavm/dependency/DependencyGraphBuilder.java

@@ -490,5 +490,19 @@ class DependencyGraphBuilder {
                     new CallLocation(caller.getMethod(), currentLocation)).use();
             currentExceptionConsumer.consume(dependencyChecker.getType("java.lang.NullPointerException"));
         }
+
+        @Override
+        public void monitorEnter(VariableReader objectRef) {
+            dependencyChecker.linkMethod(
+                    new MethodReference(Object.class, "monitorEnter", Object.class, void.class), 
+                    new CallLocation(caller.getMethod(), currentLocation)).use();
+        }
+
+        @Override
+        public void monitorExit(VariableReader objectRef) {
+            dependencyChecker.linkMethod(
+                    new MethodReference(Object.class, "monitorExit", Object.class, void.class), 
+                    new CallLocation(caller.getMethod(), currentLocation)).use();
+        }
     };
 }

teavm-core/src/main/java/org/teavm/javascript/ast/MonitorEnterStatement.java

@@ -22,7 +22,7 @@ package org.teavm.javascript.ast;
 public class MonitorEnterStatement extends Statement {
     
     private NodeLocation location;
-    private VariableExpr objectRef;
+    private Expr objectRef;
 
     @Override
     public void acceptVisitor(StatementVisitor visitor) {
@@ -46,14 +46,14 @@ public class MonitorEnterStatement extends Statement {
     /**
      * @return the objectRef
      */
-    public VariableExpr getObjectRef() {
+    public Expr getObjectRef() {
         return objectRef;
     }
 
     /**
      * @param objectRef the objectRef to set
      */
-    public void setObjectRef(VariableExpr objectRef) {
+    public void setObjectRef(Expr objectRef) {
         this.objectRef = objectRef;
     }
     

teavm-core/src/main/java/org/teavm/javascript/ast/MonitorExitStatement.java

@@ -22,7 +22,7 @@ package org.teavm.javascript.ast;
 public class MonitorExitStatement extends Statement {
 
     private NodeLocation location;
-    private VariableExpr objectRef;
+    private Expr objectRef;
     
     @Override
     public void acceptVisitor(StatementVisitor visitor) {
@@ -46,14 +46,14 @@ public class MonitorExitStatement extends Statement {
     /**
      * @return the objectRef
      */
-    public VariableExpr getObjectRef() {
+    public Expr getObjectRef() {
         return objectRef;
     }
 
     /**
      * @param objectRef the objectRef to set
      */
-    public void setObjectRef(VariableExpr objectRef) {
+    public void setObjectRef(Expr objectRef) {
         this.objectRef = objectRef;
     }
     

teavm-core/src/main/java/org/teavm/model/instructions/InstructionReader.java

@@ -100,4 +100,8 @@ public interface InstructionReader {
     void initClass(String className);
 
     void nullCheck(VariableReader receiver, VariableReader value);
+    
+    void monitorEnter(VariableReader objectRef);
+    
+    void monitorExit(VariableReader objectRef);
 }

teavm-core/src/main/java/org/teavm/model/util/InstructionStringifier.java

@@ -352,4 +352,14 @@ public class InstructionStringifier implements InstructionReader {
     public void nullCheck(VariableReader receiver, VariableReader value) {
         sb.append("@").append(receiver.getIndex()).append(" := nullCheck @").append(value.getIndex());
     }
+
+    @Override
+    public void monitorEnter(VariableReader objectRef) {
+        sb.append("monitorenter @").append(objectRef.getIndex());
+    }
+
+    @Override
+    public void monitorExit(VariableReader objectRef) {
+        sb.append("monitorexit @").append(objectRef.getIndex());
+    }
 }

teavm-core/src/main/java/org/teavm/model/util/ProgramUtils.java

@@ -495,5 +495,21 @@ public final class ProgramUtils {
             copy = insnCopy;
             copy.setLocation(location);
         }
+
+        @Override
+        public void monitorEnter(VariableReader objectRef) {
+            MonitorEnterInstruction insnCopy = new MonitorEnterInstruction();
+            insnCopy.setObjectRef(copyVar(objectRef));
+            copy = insnCopy;
+            copy.setLocation(location);
+        }
+
+        @Override
+        public void monitorExit(VariableReader objectRef) {
+            MonitorExitInstruction insnCopy = new MonitorExitInstruction();
+            insnCopy.setObjectRef(copyVar(objectRef));
+            copy = insnCopy;
+            copy.setLocation(location);
+        }
     }
 }

teavm-core/src/main/java/org/teavm/tooling/ProgramSourceAggregator.java

@@ -88,4 +88,14 @@ class ProgramSourceAggregator implements InstructionReader {
     @Override public void isInstance(VariableReader receiver, VariableReader value, ValueType type) { }
     @Override public void initClass(String className) { }
     @Override public void nullCheck(VariableReader receiver, VariableReader value) { }
+
+    @Override
+    public void monitorEnter(VariableReader objectRef) {
+        
+    }
+
+    @Override
+    public void monitorExit(VariableReader objectRef) {
+        
+    }
 }