Allow application of external mixins to generated dart protos.

test/all_tests.dart

Index: test/all_tests.dart
diff --git a/test/all_tests.dart b/test/all_tests.dart
index c0e1e9d41c096ad3ba44042f4e34b8f6ae837670..3e6cbf93a44d38a14dbb18aaf7c18e5366190877 100755
--- a/test/all_tests.dart
+++ b/test/all_tests.dart
@@ -18,6 +18,7 @@ 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;
@@ -40,6 +41,7 @@ void main() {
   map.main();
   message_generator.main();
   message.main();
+  mixin.main();
   protoc_options.main();
   repeated_field.main();
   service.main();