Extend the mDNS package with a native extension used on Mac OS

pkg/mdns/test/native_extension_test.dart

+// Copyright (c) 2015, the Fletch 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.md file.
+
+import 'dart:async';
+
+import 'package:expect/expect.dart';
+import 'package:mdns/src/native_extension_client.dart';
+
+Future<Null> main() async {
+  String message1 = 'Hello, world!';
+  String result1 = await nativeExtensionEchoTest(message1);
+  Expect.equals(message1, result1);
+
+  List<int> message2 = [1, 2];
+  List<int> result2 = await nativeExtensionEchoTest(message2);
+  Expect.listEquals(message2, result2);
+}