Don't call _ensureRequestAnimationFrame in Dartium.

tools/dom/templates/html/impl/impl_Window.darttemplate

 // 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.
 
 part of $LIBRARYNAME;
 
 typedef void RemoveFrameRequestMapping(int id);
 
 /**
  * The task object representing animation-frame requests.
@@ skipping 228 matching lines @@
   /**
    * Cancels an animation frame request.
    *
    * ## Other resources
    *
    * * [Window.cancelAnimationFrame](https://developer.mozilla.org/en-US/docs/Web/API/Window.cancelAnimationFrame)
    *   from MDN.
    */
   @DomName('Window.cancelAnimationFrame')
   void cancelAnimationFrame(int id) {
+$if DART2JS
     _ensureRequestAnimationFrame();
+$endif
     var task = AnimationFrameTask._tasks.remove(id);
     if (task == null) {
       // Assume that the animation frame request wasn't intercepted by a zone.
       _cancelAnimationFrame(id);
       return;
     }
     task.cancel(this);
   }
 
 $if DART2JS
@@ skipping 219 matching lines @@
       {bool useCapture: false}) {
 $if DART2JS
     // Specify the generic type for _ElementEventStreamImpl only in dart2js to
     // avoid checked mode errors in dartium.
     return new _ElementListEventStreamImpl<BeforeUnloadEvent>(e, _eventType, useCapture);
 $else
     return new _ElementListEventStreamImpl(e, _eventType, useCapture);
 $endif
   }
 }