Do not treat callbacks with no handlers as interfaces.

lib/dom/frog/dom_frog.dart

Index: lib/dom/frog/dom_frog.dart
diff --git a/lib/dom/frog/dom_frog.dart b/lib/dom/frog/dom_frog.dart
index 88b77ca828d61841fd49c53ff9467ff167056e57..b090a8ccd3e5ed16f38c8bad54becd7cb9a8f41b 100644
--- a/lib/dom/frog/dom_frog.dart
+++ b/lib/dom/frog/dom_frog.dart
@@ -5230,9 +5230,6 @@ class _MouseEventJs extends _UIEventJs implements MouseEvent native "*MouseEvent
   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;
@@ -17989,14 +17986,6 @@ 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;