Do not treat callbacks with no handlers as interfaces.

lib/dom/frog/dom_frog.dart

 #library('dom');
 
 // Copyright (c) 2011, 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.
 
 // DO NOT EDIT
 // Auto-generated Dart DOM library.
 
 
@@ skipping 5212 matching lines @@
 
   final int webkitMovementY;
 
   final int x;
 
   final int y;
 
   void initMouseEvent(String type, bool canBubble, bool cancelable, _DOMWindowJs view, int detail, int screenX, int screenY, int clientX, int clientY, bool ctrlKey, bool altKey, bool shiftKey, bool metaKey, int button, _EventTargetJs relatedTarget) native;
 }
 
-class _MutationCallbackJs extends _DOMTypeJs implements MutationCallback native "*MutationCallback" {
-}
-
 class _MutationEventJs extends _EventJs implements MutationEvent native "*MutationEvent" {
 
   static final int ADDITION = 2;
 
   static final int MODIFICATION = 1;
 
   static final int REMOVAL = 3;
 
   final int attrChange;
 
@@ skipping 12736 matching lines @@
   final int y;
 
   void initMouseEvent(String type, bool canBubble, bool cancelable, DOMWindow view, int detail, int screenX, int screenY, int clientX, int clientY, bool ctrlKey, bool altKey, bool shiftKey, bool metaKey, int button, EventTarget relatedTarget);
 }
 // 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 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;
 
   static final int MODIFICATION = 1;
 
   static final int REMOVAL = 3;
 
   final int attrChange;
 
@@ skipping 7252 matching lines @@
     if (length < 0) throw new IllegalArgumentException('length');
     if (start < 0) throw new IndexOutOfRangeException(start);
     int end = start + length;
     if (end > a.length) throw new IndexOutOfRangeException(end);
     for (int i = start; i < end; i++) {
       accumulator.add(a[i]);
     }
     return accumulator;
   }
 }