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

pkg/mdns/lib/src/constants.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.
 
 library mdns.src.constants;
 
 import 'dart:io';
 
 InternetAddress mDnsAddress = new InternetAddress('224.0.0.251');
 const int mDnsPort = 5353;
 
 // Offsets into the header. See https://tools.ietf.org/html/rfc1035.
 const int idOffset = 0;
 const int flagsOffset = 2;
 const int qdcountOffset = 4;
 const int ancountOffset = 6;
 const int nscountOffset = 8;
 const int arcountOffset = 10;
 
 const int headerSize = 12;
 
+const int srvHeaderSize = 6;
+
 const int responseFlags = 0x8400;
 
 class RRType {
   static const int A = 1;
   static const int AAAA = 28;
   static const int PTR = 12;
   static const int SRV = 33;
   static const int TXT = 16;
 }
 
 class RRClass {
   static const int IN = 1;
 }