Do not convert a Dart closure parameter to a JS closure if the parameter was not provided.

lib/compiler/implementation/native_emitter.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class NativeEmitter {
 
   CodeEmitterTask emitter;
   StringBuffer nativeBuffer;
 
   // Classes that participate in dynamic dispatch. These are the
@@ skipping 150 matching lines @@
 
     classesWithDynamicDispatch.add(classElement);
   }
 
   List<ClassElement> getDirectSubclasses(ClassElement cls) {
     List<ClassElement> result = directSubtypes[cls];
     return result === null ? const<ClassElement>[] : result;
   }
 
   void potentiallyConvertDartClosuresToJs(StringBuffer code,
-                                          FunctionElement member) {
+                                          FunctionElement member,
+                                          List<String> argumentsBuffer) {
     FunctionSignature parameters = member.computeSignature(compiler);
     Element converter =
         compiler.findHelper(const SourceString('convertDartClosureToJS'));
     String closureConverter = compiler.namer.isolateAccess(converter);
     parameters.forEachParameter((Element parameter) {
+      String name = parameter.name.slowToString();
+      // If [name] is not in [argumentsBuffer], then the parameter is
+      // an optional parameter that was not provided for that stub.
+      if (argumentsBuffer.indexOf(name) == -1) return;
       Type type = parameter.computeType(compiler);
       if (type is FunctionType) {
-        String name = parameter.name.slowToString();
         int arity = type.computeArity();
         code.add('  $name = $closureConverter($name, $arity);\n');
       }
     });
   }
 
   String generateParameterStub(Element member,
                                String invocationName,
                                String stubParameters,
                                List<String> argumentsBuffer,
                                int indexOfLastOptionalArgumentInParameters,
                                StringBuffer buffer) {
     // The target JS function may check arguments.length so we need to
     // make sure not to pass any unspecified optional arguments to it.
     // For example, for the following Dart method:
     //   foo([x, y, z]);
     // The call:
     //   foo(y: 1)
     // must be turned into a JS call to:
     //   foo(null, y).
 
     List<String> nativeArgumentsBuffer = argumentsBuffer.getRange(
         0, indexOfLastOptionalArgumentInParameters + 1);
 
     ClassElement classElement = member.enclosingElement;
     String nativeName = classElement.nativeName.slowToString();
     String nativeArguments = Strings.join(nativeArgumentsBuffer, ",");
 
     StringBuffer code = new StringBuffer();
-    potentiallyConvertDartClosuresToJs(code, member);
+    potentiallyConvertDartClosuresToJs(code, member, argumentsBuffer);
 
     if (!nativeMethods.contains(member)) {
       // When calling a method that has a native body, we call it
       // with our calling conventions.
       String arguments = Strings.join(argumentsBuffer, ",");
       code.add('  return this.${compiler.namer.getName(member)}($arguments)');
     } else {
       // When calling a JS method, we call it with the native name.
       String name = redirectingMethods[member];
       if (name === null) name = member.name.slowToString();
@@ skipping 174 matching lines @@
     String toStringName = compiler.namer.instanceMethodName(
         null, const SourceString('toString'), 0);
     objectProperties.add("$defPropName(Object.prototype, '$toStringName', ");
     objectProperties.add(
         'function() { return $toStringHelperName(this); });\n');
 
     targetBuffer.add('$defineNativeClassName = $defineNativeClassFunction;\n');
     targetBuffer.add('$objectProperties$nativeBuffer\n');
   }
 }