Maintain referential integrity of proxy instances in dart:js

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 139 matching lines @@
 main() {
   _injectJs();
   useHtmlConfiguration();
 
   test('context instances should be identical', () {
     var c1 = context;
     var c2 = context;
     expect(identical(c1, c2), isTrue);
   });
 
+  test('identical JS objects should have identical proxies', () {
+    var o1 = context['location'];
+    var o2 = context['location'];
+    expect(identical(o1, o2), isTrue);
+  });
+
+  test('identical JS functions should have identical proxies', () {
+    var f1 = context['Object'];
+    var f2 = context['Object'];
+    expect(identical(f1, f2), 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);
   });
 
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     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));
   });
 }