Add a new implementation of HashMap that uses JS objects for its (multiple) hash tables.

sdk/lib/_internal/compiler/implementation/lib/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:_interceptors';
 import 'dart:_js_helper' show checkNull,
                               getRuntimeTypeString,
                               isJsArray,
                               JSSyntaxRegExp,
                               Primitives,
                               TypeImpl,
                               stringJoinUnchecked;
 
 patch void print(var object) {
@@ skipping 275 matching lines @@
 
 patch class StackTrace {
   patch String get fullStackTrace {
     throw new UnsupportedError('fullStackTrace');
   }
 
   patch String get stackTrace {
     throw new UnsupportedError('stackTrace');
   }
 }