Improve resource record implementation in the mdns package.

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';
+
+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();
+}