Update the mDNS native extension for Mac OS to handle PTR and SRV records

pkg/mdns/bin/mdns-sd.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.
 
 // Example script to illustrate how to use the mdns package to discover services
 // on the local network.
 
-import 'mdns.dart';
+import '../lib/mdns.dart';
 
 main(List<String> args) async {
   if (args.length != 1) {
     print('''
 Please provide the name of a service as argument.
 
 For example:
   dart mdns-sd.dart _workstation._tcp.local''');
     return;
   }
   MDnsClient client = new MDnsClient();
   await client.start();
   await for (ResourceRecord ptr in client.lookup(RRType.PTR, args[0])) {
     String domain = ptr.domainName;
     await for (ResourceRecord srv in client.lookup(RRType.SRV, domain)) {
       String target = srv.target;
       await for (ResourceRecord ip in client.lookup(RRType.A, target)) {
         print('Service instance found at $target ($ip).');
       }
     }
   }
   client.stop();
 }