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

lib/dom/templates/html/dart2js/factoryprovider_FormData.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.
 
 class $FACTORYPROVIDER {
-  factory HTMLAudioElement([String src = null]) native
-'''
-if (src == null) return new Audio();
-return new Audio(src);
-''';
+  factory FormData([FormElement form = null]) native '''
+    if (form == null) return new FormData();
+    return new FormData(form);
+  ''';
 }