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

tests/html/js_test.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 jsTest;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:js';
 
@@ skipping 133 matching lines @@
   static final BLUE = new Color._("blue");
   String _value;
   Color._(this._value);
   String toJs() => this._value;
 }
 
 main() {
   _injectJs();
   useHtmlConfiguration();
 
+  test('context instances should be identical', () {
+    var c1 = context;
+    var c2 = context;
+    expect(identical(c1, c2), isTrue);
+  });
+
   test('read global field', () {
     expect(context['x'], equals(42));
     expect(context['y'], isNull);
   });
 
   test('read global field with underscore', () {
     expect(context['_x'], equals(123));
     expect(context['y'], isNull);
   });
 
@@ skipping 296 matching lines @@
     context['dartDate'] = date;
     expect(context['dartDate'], equals(date));
   });
 
   test('usage of Serializable', () {
     final red = Color.RED;
     context['color'] = red;
     expect(context['color'], equals(red._value));
   });
 }