Properly generate MutationCallback in dart:html.

lib/html/dartium/html_dartium.dart

Index: lib/html/dartium/html_dartium.dart
diff --git a/lib/html/dartium/html_dartium.dart b/lib/html/dartium/html_dartium.dart
index 3993404a7272d1f7bceb7cf9f55d10eebc0abe74..65cf31053195ba2e892dc40d31522d22e58daaba 100644
--- a/lib/html/dartium/html_dartium.dart
+++ b/lib/html/dartium/html_dartium.dart
@@ -13166,15 +13166,6 @@ class _MouseEventImpl extends _UIEventImpl implements MouseEvent {
 
 // WARNING: Do not edit - generated code.
 
-class _MutationCallbackImpl extends NativeFieldWrapperClass1 implements MutationCallback {
-
-}
-// 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.
-
-// WARNING: Do not edit - generated code.
-
 class _MutationEventImpl extends _EventImpl implements MutationEvent {
 
   int get attrChange() native "MutationEvent_attrChange_Getter";
@@ -30747,9 +30738,7 @@ interface MouseEvent extends UIEvent default _MouseEventFactoryProvider {
 
 // WARNING: Do not edit - generated code.
 
-/// @domName MutationCallback
-interface MutationCallback {
-}
+typedef bool MutationCallback(List<MutationRecord> mutations, WebKitMutationObserver observer);
 // 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.