Allow application of external mixins to generated dart protos.

test/all_tests.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 protoc_plugin_all_tests;
 
 import 'bazel_test.dart' as bazel;
 import 'client_generator_test.dart' as client_generator;
 import 'const_generator_test.dart' as const_generator;
 import 'enum_generator_test.dart' as enum_generator;
 import 'extension_test.dart' as extension;
 import 'file_generator_test.dart' as file_generator;
 import 'generated_message_test.dart' as generated_message;
 import 'hash_code_test.dart' as hash_code;
 import 'indenting_writer_test.dart' as indenting_writer;
 import 'json_test.dart' as json;
 import 'map_test.dart' as map;
 import 'message_generator_test.dart' as message_generator;
 import 'message_test.dart' as message;
+import 'mixin_test.dart' as mixin;
 import 'protoc_options_test.dart' as protoc_options;
 import 'repeated_field_test.dart' as repeated_field;
 import 'service_test.dart' as service;
 import 'service_generator_test.dart' as service_generator;
 import 'unknown_field_set_test.dart' as unknown_field_set;
 import 'validate_fail_test.dart' as validate_fail;
 import 'wire_format_test.dart' as wire_format;
 
 void main() {
   bazel.main();
   client_generator.main();
   const_generator.main();
   enum_generator.main();
   extension.main();
   file_generator.main();
   generated_message.main();
   hash_code.main();
   indenting_writer.main();
   json.main();
   map.main();
   message_generator.main();
   message.main();
+  mixin.main();
   protoc_options.main();
   repeated_field.main();
   service.main();
   service_generator.main();
   unknown_field_set.main();
   validate_fail.main();
   wire_format.main();
 }