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

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.
 
 /**
  * Support for interoperating with JavaScript.
  * 
  * This library provides access to JavaScript objects from Dart, allowing
  * Dart code to get and set properties, and call methods of JavaScript objects
  * and invoke JavaScript functions. The library takes care of converting
@@ skipping 453 matching lines @@
       JS('void', 'Object.defineProperty(#, #, { value: #})', o, name, value);
       return true;
     } catch(e) {
       // object is native and lies about being extensible
       // see https://bugzilla.mozilla.org/show_bug.cgi?id=775185
     }
   }
   return false;
 }
 
+Object _getOwnProperty(o, String name) {
+  if (JS('bool', 'Object.prototype.hasOwnProperty.call(#, #)', o, name)) {
+    return JS('', '#[#]', o, name);
+  }
+  return null;
+}
+
 bool _isLocalObject(o) => JS('bool', '# instanceof Object', o);
 
 dynamic _convertToJS(dynamic o) {
   if (o == null) {
     return null;
   } else if (o is String || o is num || o is bool
       || o is Blob || o is Event || o is KeyRange || o is ImageData
       || o is Node || o is TypedData || o is Window) {
     return o;
   } else if (o is DateTime) {
     return Primitives.lazyAsJsDate(o);
   } else if (o is JsObject) {
     return o._jsObject;
   } else if (o is Function) {
     return _getJsProxy(o, _JS_FUNCTION_PROPERTY_NAME, (o) {
       var jsFunction = _convertDartFunction(o);
       // set a property on the JS closure referencing the Dart closure
       _defineProperty(jsFunction, _DART_CLOSURE_PROPERTY_NAME, o);
       return jsFunction;
     });
   } else {
     return _getJsProxy(o, _JS_OBJECT_PROPERTY_NAME,
         (o) => JS('', 'new DartObject(#)', o));
   }
 }
 
 Object _getJsProxy(o, String propertyName, createProxy(o)) {
-  var jsProxy = JS('', '#[#]', o, propertyName);
+  var jsProxy = _getOwnProperty(o, propertyName);
   if (jsProxy == null) {
     jsProxy = createProxy(o);
     _defineProperty(o, propertyName, jsProxy);
   }
   return jsProxy;
 }
 
 // converts a Dart object to a reference to a native JS object
 // which might be a DartObject JS->Dart proxy
 Object _convertToDart(o) {
@@ skipping 24 matching lines @@
   } else if (JS('bool', '# instanceof Array', o)) {
     return _getDartProxy(o, _DART_OBJECT_PROPERTY_NAME,
         (o) => new JsArray._fromJs(o));
   } else {
     return _getDartProxy(o, _DART_OBJECT_PROPERTY_NAME,
         (o) => new JsObject._fromJs(o));
   }
 }
 
 Object _getDartProxy(o, String propertyName, createProxy(o)) {
-  var dartProxy = JS('', '#[#]', o, propertyName);
+  var dartProxy = _getOwnProperty(o, propertyName);
   // Temporary fix for dartbug.com/15193
   // In some cases it's possible to see a JavaScript object that
   // came from a different context and was previously proxied to
   // Dart in that context. The JS object will have a cached proxy
   // but it won't be a valid Dart object in this context.
   // For now we throw away the cached proxy, but we should be able
   // to cache proxies from multiple JS contexts and Dart isolates.
   if (dartProxy == null || !_isLocalObject(o)) {
     dartProxy = createProxy(o);
     _defineProperty(o, propertyName, dartProxy);
   }
   return dartProxy;
 }