Allow application of external mixins to generated dart protos.

test/message_generator_test.dart

 #!/usr/bin/env dart
 // Copyright (c) 2013, 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.
 
 library message_generator_test;
 
 import 'package:protoc_plugin/indenting_writer.dart';
 import 'package:protoc_plugin/protoc.dart';
+import 'package:test/test.dart';
+
 import 'package:protoc_plugin/src/descriptor.pb.dart';
 import 'package:protoc_plugin/src/plugin.pb.dart';
-import 'package:test/test.dart';
 
 void main() {
   test('testMessageGenerator', () {
     // NOTE: Below > 80 cols because it is matching generated code > 80 cols.
     String expectedEnums = r'''
 class PhoneNumber_PhoneType extends ProtobufEnum {
   static const PhoneNumber_PhoneType MOBILE = const PhoneNumber_PhoneType._(0, 'MOBILE');
   static const PhoneNumber_PhoneType HOME = const PhoneNumber_PhoneType._(1, 'HOME');
   static const PhoneNumber_PhoneType WORK = const PhoneNumber_PhoneType._(2, 'WORK');
 
@@ skipping 97 matching lines @@
           ..number = 3
           ..label = FieldDescriptorProto_Label.LABEL_OPTIONAL
           ..type = FieldDescriptorProto_Type.TYPE_STRING
           ..defaultValue = r'$'
       ])
       ..enumType.add(ed);
     var options = parseGenerationOptions(
         new CodeGeneratorRequest(), new CodeGeneratorResponse());
 
     FileGenerator fg = new FileGenerator(fd);
-    MessageGenerator mg = new MessageGenerator(md, fg, null);
+    MessageGenerator mg = new MessageGenerator(md, fg, {}, '');
 
     var ctx = new GenerationContext(options);
     mg.register(ctx);
     mg.resolve(ctx);
 
     var writer = new IndentingWriter();
     mg.generate(writer);
     expect(writer.toString(), expected);
 
     writer = new IndentingWriter();
     mg.generateEnums(writer);
     expect(writer.toString(), expectedEnums);
   });
 }