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

tests/corelib/map_values_test.dart

 // Copyright (c) 2011, 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.
 
 main() {
   var map1 = { "foo": 42, "bar": 499 };
   var map2 = {};
   var map3 = const { "foo": 42, "bar": 499 };
   var map4 = const {};
   var map5 = new Map<String, int>();
@@ skipping 18 matching lines @@
   Expect.equals(499, map3.values.last);
 
   Expect.isTrue(map4.values is Iterable);
   Expect.isFalse(map4.values is List);
   Expect.equals(0, map4.values.length);
 
   Expect.isTrue(map5.values is Iterable);
   Expect.isFalse(map5.values is List);
   Expect.equals(2, map5.values.length);
   Expect.isTrue(map5.values.first == 43 || map5.values.first == 500);
-  Expect.isTrue(map5.values.last == 43 || map5.values.first == 500);
+  Expect.isTrue(map5.values.last == 43 || map5.values.last == 500);
   Expect.notEquals(map5.values.first, map5.values.last);
 
   Expect.isTrue(map6.values is Iterable);
   Expect.isFalse(map6.values is List);
   Expect.equals(0, map6.values.length);
 }