Revert "Make PbMapMixin implement Map<String, dynamic>"

test/map_mixin_test.dart

Index: test/map_mixin_test.dart
diff --git a/test/map_mixin_test.dart b/test/map_mixin_test.dart
index ce0143ae8ea983d8d1ccd3d82b081a6614aba162..b732a96236a557544320f944540c6bf3350dc2b7 100644
--- a/test/map_mixin_test.dart
+++ b/test/map_mixin_test.dart
@@ -15,7 +15,7 @@ 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<String, dynamic>, PbMapMixin {
+class Rec extends MockMessage with MapMixin, PbMapMixin {
   get info_ => _info;
   static final _info = mockInfo("Rec", () => new Rec());