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

lib/dom/templates/html/dart2js/factoryprovider_FormData.darttemplate

Index: lib/dom/templates/html/dart2js/factoryprovider_FormData.darttemplate
diff --git a/lib/dom/templates/dom/dart2js/factoryprovider_HTMLAudioElement.darttemplate b/lib/dom/templates/html/dart2js/factoryprovider_FormData.darttemplate
similarity index 63%
copy from lib/dom/templates/dom/dart2js/factoryprovider_HTMLAudioElement.darttemplate
copy to lib/dom/templates/html/dart2js/factoryprovider_FormData.darttemplate
index c6288fe9b813cabd7150368a56707200ad8c623e..33d5cbf1bcece4c1843936936cbed723b11bc54e 100644
--- a/lib/dom/templates/dom/dart2js/factoryprovider_HTMLAudioElement.darttemplate
+++ b/lib/dom/templates/html/dart2js/factoryprovider_FormData.darttemplate
@@ -3,9 +3,8 @@
 // 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);
+  ''';
 }