Initial support for generating client and server stubs for Dart.

lib/protoc.dart

Index: lib/protoc.dart
diff --git a/lib/protoc.dart b/lib/protoc.dart
index 20c5dc7f1f1ee184897c84cc221931e0f2ac32e5..19fed1810c86e13e936ab63e833f492dca6b70f7 100644
--- a/lib/protoc.dart
+++ b/lib/protoc.dart
@@ -9,6 +9,7 @@ import 'package:path/path.dart' as path;
 import 'src/descriptor.pb.dart';
 import 'src/plugin.pb.dart';
 
+part 'client_generator.dart';
 part 'code_generator.dart';
 part 'enum_generator.dart';
 part 'exceptions.dart';
@@ -19,4 +20,5 @@ part 'message_generator.dart';
 part 'options.dart';
 part 'output_config.dart';
 part 'protobuf_field.dart';
+part 'service_generator.dart';
 part 'writer.dart';