Make PbMapMixin implement Map<String, dynamic>

test/map_mixin_test.dart

 #!/usr/bin/env dart
 // 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.
 
 // Unit tests for PbMapMixin.
 // There are more tests in the dart-protoc-plugin package.
 library map_mixin_test;
 
 import 'dart:collection' show MapMixin;
 
 import 'package:protobuf/src/protobuf/mixins/map_mixin.dart';
 import 'package:test/test.dart' show test, expect, same, throws;
 
 import 'mock_util.dart' show MockMessage, mockInfo;
 
 // A minimal protobuf implementation compatible with PbMapMixin.
-class Rec extends MockMessage with MapMixin, PbMapMixin {
+class Rec extends MockMessage with MapMixin<String, dynamic>, PbMapMixin {
   get info_ => _info;
   static final _info = mockInfo("Rec", () => new Rec());
 
   @override
   String toString() => "Rec(${val}, \"${str}\")";
 }
 
 main() {
   test('PbMapMixin methods return default field values', () {
     var r = new Rec();
@@ skipping 68 matching lines @@
   });
 
   test("reading protobuf values shouldn't change equality", () {
     var a = new Rec();
     var b = new Rec();
     expect(a == b, true);
     new Map.from(a);
     expect(a == b, true);
   });
 }