Wrap Window in case it's a window from a different page / frame.

lib/html/templates/html/dart2js/impl_LocalWindow.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 native "@*DOMWindow" {
 
   _DocumentImpl get document() native "return this.document;";
 
-  Window get _top() native "return this.top;";
-
-  // Override top to return secure wrapper.
-  Window get top => _DOMWindowCrossFrameImpl._createSafe(_top);
-
   Window _open2(url, name) native "return this.open(url, name);";
 
   Window _open3(url, name, options) native "return this.open(url, name, options);";
 
   Window open(String url, String name, [String options]) {
     if (options == null) {
       return _DOMWindowCrossFrameImpl._createSafe(_open2(url, name));
     } else {
       return _DOMWindowCrossFrameImpl._createSafe(_open3(url, name, options));
     }
@@ skipping 109 matching lines @@
 
   String createObjectUrl(object) native '''
     return (window.URL || window.webkitURL).createObjectURL(object)
   ''';
 
   void revokeObjectUrl(String objectUrl) native '''
     (window.URL || window.webkitURL).revokeObjectURL(objectUrl)
   ''';
 $!MEMBERS
 }