Initial support for generating client and server stubs for Dart.

test/file_generator_test.dart

Index: test/file_generator_test.dart
diff --git a/test/file_generator_test.dart b/test/file_generator_test.dart
index 78f537e0ba51087ce4bc6fd352a0c1d9820ad75b..b65a609edb3a58ecb4dea881ca363b7709518af6 100644
--- a/test/file_generator_test.dart
+++ b/test/file_generator_test.dart
@@ -74,6 +74,8 @@ void main() {
 ///
 library test;
 
+import 'dart:async';
+
 import 'package:fixnum/fixnum.dart';
 import 'package:protobuf/protobuf.dart';
 
@@ -128,6 +130,8 @@ class PhoneNumber extends GeneratedMessage {
 ///
 library test;
 
+import 'dart:async';
+
 import 'package:fixnum/fixnum.dart';
 import 'package:protobuf/protobuf.dart';
 
@@ -201,6 +205,8 @@ class PhoneNumber extends GeneratedMessage {
 ///
 library pb_library;
 
+import 'dart:async';
+
 import 'package:fixnum/fixnum.dart';
 import 'package:protobuf/protobuf.dart';
 
@@ -256,6 +262,8 @@ class PhoneNumber extends GeneratedMessage {
 ///
 library test;
 
+import 'dart:async';
+
 import 'package:fixnum/fixnum.dart';
 import 'package:protobuf/protobuf.dart';
 
@@ -314,6 +322,8 @@ class PhoneNumber extends GeneratedMessage {
 ///
 library test;
 
+import 'dart:async';
+
 import 'package:fixnum/fixnum.dart';
 import 'package:protobuf/protobuf.dart';
 import 'package1.pb.dart' as p1;