Do not treat callbacks with no handlers as interfaces.

lib/html/frog/html_frog.dart

Index: lib/html/frog/html_frog.dart
diff --git a/lib/html/frog/html_frog.dart b/lib/html/frog/html_frog.dart
index 892aacf34c2ac026eb9a23f13a4e54c382f4a476..2a22186123002ce2777cef1b4270402b9d85760d 100644
--- a/lib/html/frog/html_frog.dart
+++ b/lib/html/frog/html_frog.dart
@@ -9116,9 +9116,6 @@ class _MouseEventImpl extends _UIEventImpl implements MouseEvent native "*MouseE
   void $dom_initMouseEvent(String type, bool canBubble, bool cancelable, _WindowImpl view, int detail, int screenX, int screenY, int clientX, int clientY, bool ctrlKey, bool altKey, bool shiftKey, bool metaKey, int button, _EventTargetImpl relatedTarget) native "initMouseEvent";
 }
 
-class _MutationCallbackImpl implements MutationCallback native "*MutationCallback" {
-}
-
 class _MutationEventImpl extends _EventImpl implements MutationEvent native "*MutationEvent" {
 
   static final int ADDITION = 2;
@@ -26808,15 +26805,6 @@ interface MouseEvent extends UIEvent default _MouseEventFactoryProvider {
 
 // WARNING: Do not edit - generated code.
 
-/// @domName MutationCallback
-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.
-
 /// @domName MutationEvent
 interface MutationEvent extends Event {