Add a mDNS package

pkg/mdns/lib/src/constants.dart

+// Copyright (c) 2015, the Dart 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.
+
+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 responseFlags = 0x8400;
+
+const int ipV4AddressType = 0x0001;
+const int ipV4Class = 0x8001;
+const int ipV6AddressType = 0x001c;
+const int ipV6Class = 0x8001;