Ensure that dart:js does not attempt to load cached proxies from prototypes.

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:typed_data' show ByteBuffer, Int32List;
 import 'dart:indexed_db' show IdbFactory, KeyRange;
@@ skipping 168 matching lines @@
 function Liar(){}
 
 Liar.prototype.toString = function() {
   return 1;
 }
 
 function identical(o1, o2) {
   return o1 === o2;
 }
 
+var someProto = { role: "proto" };
+var someObject = Object.create(someProto);
+someObject.role = "object";
+
 """;
   document.body.append(script);
 }
 
 // Some test are either causing other test to fail in IE9, or they are failing
 // for unknown reasons
 // useHtmlConfiguration+ImageData bug: dartbug.com/14355
 skipIE9_test(String description, t()) {
   if (Platform.supportsTypedData) {
     test(description, t);
@@ skipping 80 matching lines @@
       context.deleteProperty('foo2');
     });
 
     test('retrieve same dart Object', () {
       final obj = new Object();
       context['obj'] = obj;
       expect(context['obj'], same(obj));
       context.deleteProperty('obj');
     });
 
+    group('caching', () {
+      test('JS->Dart', () {
+        // Test that we are not pulling cached proxy from the prototype
+        // when asking for a proxy for the object.
+        final proto = context['someProto'];
+        expect(proto['role'], same('proto'));
+        final obj = context['someObject'];
+        expect(obj['role'], same('object'));
+      });
+    });
+
   });
 
   group('context', () {
 
     test('read global field', () {
       expect(context['x'], equals(42));
       expect(context['y'], isNull);
     });
 
     test('read global field with underscore', () {
@@ skipping 689 matching lines @@
           // TODO(jacobr): make this test pass. Currently some type information
           // is lost when typed arrays are passed between JS and Dart.
           // expect(context.callMethod('isPropertyInstanceOf', ['o', listType]),
           //    isTrue);
           context.deleteProperty('o');
         }
       });
 
     });
   });
+
 }