Re-land "Extend the mDNS package with a native extension used on Mac OS"

pkg/mdns/test/lookup_resolver_test.dart

-// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// 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 file.
+// BSD-style license that can be found in the LICENSE.md file.
 
 import 'dart:io';
 
 import 'package:expect/expect.dart';
 import 'package:mdns/src/packet.dart';
 import 'package:mdns/src/lookup_resolver.dart';
 
 main() async {
   await testTimeout();
   await testResult();
@@ skipping 46 matching lines @@
   resolver.handleResponse([response0]);
   var response2 = new DecodeResult('yyy', new InternetAddress('2.3.4.5'));
   resolver.handleResponse([response0]);
   resolver.handleResponse([response2, response1]);
   resolver.handleResponse([response0]);
   var result1 = await futureResult1;
   var result2 = await futureResult2;
   Expect.equals('1.2.3.4', result1.address);
   Expect.equals('2.3.4.5', result2.address);
 }