fixes #168, dart:js implementation with a test

tool/input_sdk/patch/core_patch.dart

 // Copyright (c) 2012, 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.
 
 // Patch file for dart:core classes.
 import "dart:_internal" as _symbol_dev;
 import 'dart:_interceptors';
 import 'dart:_js_helper' show patch,
                               checkInt,
                               getRuntimeType,
@@ skipping 48 matching lines @@
   static Map<String, dynamic> _toMangledNames(
       Map<Symbol, dynamic> namedArguments) {
     Map<String, dynamic> result = {};
     namedArguments.forEach((symbol, value) {
       result[_symbolToString(symbol)] = value;
     });
     return result;
   }
 }
 
+// TODO(jmesserly): switch to WeakMap
 // Patch for Expando implementation.
 @patch
 class Expando<T> {
   @patch
   Expando([String name]) : this.name = name;
 
   @patch
   T operator[](Object object) {
     var values = Primitives.getProperty(object, _EXPANDO_PROPERTY_NAME);
     return (values == null) ? null : Primitives.getProperty(values, _getKey());
@@ skipping 365 matching lines @@
   @patch
   static bool get _isWindows => false;
 
   @patch
   static Uri get base {
     String uri = Primitives.currentUri();
     if (uri != null) return Uri.parse(uri);
     throw new UnsupportedError("'Uri.base' is not supported");
   }
 }