Nuke custom factory providers.

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;
 
 /// @domName $DOMNAME
 @SupportedBrowser(SupportedBrowser.CHROME)
 @SupportedBrowser(SupportedBrowser.FIREFOX)
 @SupportedBrowser(SupportedBrowser.SAFARI)
@@ skipping 77 matching lines @@
 $endif
 
 $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) {
+    // 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
 }