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

tests/corelib/map_keys_test.dart

Index: tests/corelib/map_keys_test.dart
diff --git a/tests/corelib/map_keys_test.dart b/tests/corelib/map_keys_test.dart
index 9297f03e060463cfccf0c6204cae4444bbd7fb05..8b3a83348f6717ce72bc9d2d69a8e4d25c6392d3 100644
--- a/tests/corelib/map_keys_test.dart
+++ b/tests/corelib/map_keys_test.dart
@@ -36,7 +36,7 @@ main() {
   Expect.isFalse(map5.keys is List);
   Expect.equals(2, map5.keys.length);
   Expect.isTrue(map5.keys.first == "foo" || map5.keys.first == "bar");
-  Expect.isTrue(map5.keys.last == "foo" || map5.keys.first == "bar");
+  Expect.isTrue(map5.keys.last == "foo" || map5.keys.last == "bar");
   Expect.notEquals(map5.keys.first, map5.keys.last);
 
   Expect.isTrue(map6.keys is Iterable);