Do not treat callbacks with no handlers as interfaces.

lib/dom/dom.dart

Index: lib/dom/dom.dart
diff --git a/lib/dom/dom.dart b/lib/dom/dom.dart
index 480bf5b4b4c47277dd3330ea5c727cecd98ff4cf..669b79bf52363849924057da9e4722682305b219 100644
--- a/lib/dom/dom.dart
+++ b/lib/dom/dom.dart
@@ -6012,14 +6012,6 @@ interface MouseEvent extends UIEvent {
 
 // WARNING: Do not edit - generated code.
 
-interface 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.
-
 interface MutationEvent extends Event {
 
   static final int ADDITION = 2;