Reapply revision 11570, custom callbacks are now implemented in dartium.

lib/dom/src/dart2js_FactoryProviders.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 _AudioContextFactoryProvider {
 
   factory AudioContext() native '''
     var constructor = window.AudioContext || window.webkitAudioContext;
     return new constructor();
 ''';
 }
 
+class _FormDataFactoryProvider {
+
+  factory FormData([HTMLFormElement form = null]) native '''
+    if (form == null) return new FormData();
+    return new FormData(form);
+  ''';
+
+}
+
 class _WebKitPointFactoryProvider {
 
   factory WebKitPoint(num x, num y) native '''return new WebKitPoint(x, y);''';
 }
 
 class _WebSocketFactoryProvider {
 
   factory WebSocket(String url) native '''return new WebSocket(url);''';
 }