Cache the context proxy. Don't use window for global 'this'.

sdk/lib/js/dart2js/js_dart2js.dart

 // Copyright (c) 2013, 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.
 
 library dart.js;
 
 import 'dart:_foreign_helper' show JS;
-import 'dart:_js_helper' show convertDartClosureToJS;
+import 'dart:_js_helper' show Primitives, convertDartClosureToJS;
 
-JsObject get context {
-  return new JsObject._fromJs(JS('=Object', 'window'));
-}
+final JsObject context = new JsObject._fromJs(Primitives.computeGlobalThis());
 
 JsObject jsify(dynamic data) => data == null ? null : new JsObject._json(data);
 
 class Callback implements Serializable<JsFunction> {
   final Function _f; // here to allow capture in closure
   final bool _withThis; // here to allow capture in closure
   dynamic _jsFunction;
 
   Callback._(this._f, this._withThis) {
     _jsFunction = JS('=Object', r'''
@@ skipping 169 matching lines @@
       JS('bool', 'typeof # == "boolean" || # instanceof Boolean', o, o)) {
     return o;
   } else if (JS('bool', '# instanceof Function', o)) {
     return new JsFunction._fromJs(JS('=Object', '#', o));
   } else if (JS('bool', '# instanceof DartProxy', o)) {
     return JS('var', '#.o', o);
   } else {
     return new JsObject._fromJs(JS('=Object', '#', o));
   }
 }