fixes #216, JSNumber members

lib/runtime/dart_runtime.js

 // Copyright (c) 2015, 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.
 
 var dart, dartx;
 (function (dart) {
   'use strict';
 
   const defineProperty = Object.defineProperty;
   const getOwnPropertyDescriptor = Object.getOwnPropertyDescriptor;
@@ skipping 781 matching lines @@
    * These are the only properties safe to copy onto an existing public
    * JavaScript class.
    */
   function registerExtension(jsType, dartExtType) {
     let extProto = dartExtType.prototype;
     let jsProto = jsType.prototype;
 
     // Mark the JS type's instances so we can easily check for extensions.
     assert(jsProto[_extensionType] === void 0);
     jsProto[_extensionType] = extProto;
-    copyPropertiesHelper(jsProto, extProto, getOwnPropertySymbols(extProto));
+
+    let dartObjProto = core.Object.prototype;
+    while (extProto !== dartObjProto && extProto !== jsProto) {
+      copyPropertiesHelper(jsProto, extProto, getOwnPropertySymbols(extProto));
+      extProto = extProto.__proto__;
+    }
   }
   dart.registerExtension = registerExtension;
 
   /**
    * Mark a concrete type as implementing extension methods.
    * For example: `class MyIter implements Iterable`.
    *
    * This takes a list of names, which are the extension methods implemented.
    * It will add a forwarder, so the extension method name redirects to the
    * normal Dart method name. For example:
@@ skipping 616 matching lines @@
 
     // TODO(vsm): This is referenced (as init.globalState) from
     // isolate_helper.dart.  Where should it go?
     // See: https://github.com/dart-lang/dev_compiler/issues/164
     dart.globalState = null;
 
     /** Dart extension members. */
     dartx = dartx || {};
   }
 })(dart || (dart = {}));