Browser compat for window.requestAnimationFrame

lib/dom/templates/dom/frog/impl_DOMWindow.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.
 
 class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
-$!MEMBERS
 
   Window get _top() native "return this.top;";
 
   // Override top to return secure wrapper.
   Window get top() => _DOMWindowCrossFrameImpl._createSafe(_top);
+
+  int requestAnimationFrame(RequestAnimationFrameCallback callback) native '''
+    if (!window.requestAnimationFrame) {
+      window.requestAnimationFrame =
+          window.webkitRequestAnimationFrame ||
+          window.mozRequestAnimationFrame ||
+          window.msRequestAnimationFrame ||
+          window.oRequestAnimationFrame ||
+          function (callback) {
+            window.setTimeout(callback, 16 /* 16ms ~= 60fps */);
+          };
+    }
+    return window.requestAnimationFrame(callback);
+''';
+
+  // Protect member 'requestAnimationFrame'.
+  _requestAnimationFrame() native 'requestAnimationFrame';
+
+$!MEMBERS
 }