Support overloaded constructors.

tools/dom/templates/html/impl/impl_MutationObserver.darttemplate

 // 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.
 
 part of html;
 
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
   /**
    * Checks to see if the mutation observer API is supported on the current
@@ skipping 73 matching lines @@
 
 $if DART2JS
   static _createDict() => JS('var', '{}');
   static _add(m, String key, value) { JS('void', '#[#] = #', m, key, value); }
   static _fixupList(list) => list;  // TODO: Ensure is a JavaScript Array.
 
   // Call native function with no conversions.
   @JSName('observe')
   void _call(target, options) native;
 
-  static MutationObserver _create(MutationCallback callback) {
+  factory MutationObserver(MutationCallback callback) {
     // Dummy statement to mark types as instantiated.
     JS('MutationObserver|MutationRecord', '0');
 
     return JS('MutationObserver',
         'new(window.MutationObserver||window.WebKitMutationObserver||'
         'window.MozMutationObserver)(#)',
         convertDartClosureToJS(callback, 2));
   }
 $endif
 }