Incremental compiler: support optional arguments.

dart/sdk/lib/_internal/compiler/js_lib/js_helper.dart

 // Copyright (c) 2013, 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.
 
 library _js_helper;
 
 import 'shared/embedded_names.dart' show
     ALL_CLASSES,
     GET_ISOLATE_TAG,
     INTERCEPTED_NAMES,
@@ skipping 1937 matching lines @@
 
     // function tmp() {};
     // tmp.prototype = BC.prototype;
     // var proto = new tmp;
     // for each computed prototype property:
     //   proto[property] = ...;
     // proto._init = BC;
     // var dynClosureConstructor =
     //     new Function('self', 'target', 'receiver', 'name',
     //                  'this._init(self, target, receiver, name)');
-    // proto.constructor = dynClosureConstructor; // Necessary?
+    // proto.constructor = dynClosureConstructor;
     // dynClosureConstructor.prototype = proto;
     // return dynClosureConstructor;
 
     // We need to create a new subclass of either TearOffClosure or
     // BoundClosure.  For this, we need to create an object whose prototype is
     // the prototype is either TearOffClosure.prototype or
     // BoundClosure.prototype, respectively in pseudo JavaScript code. The
     // simplest way to access the JavaScript construction function of a Dart
     // class is to create an instance and access its constructor property.  The
     // newly created instance could in theory be used directly as the
@@ skipping 1433 matching lines @@
   throw new MainError("No top-level function named 'main'.");
 }
 
 void badMain() {
   throw new MainError("'main' is not a function.");
 }
 
 void mainHasTooManyParameters() {
   throw new MainError("'main' expects too many parameters.");
 }