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

src/pkg/mdns/mdns_extension.h

+// 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.
+
+#ifndef MDNS_EXTENSION_H_
+#define MDNS_EXTENSION_H_
+
+#include "include/dart_api.h"
+#include "include/dart_native_api.h"
+
+// Requests Ids. This should be aligned with the Dart code.
+enum RequestType {
+  kEchoRequest = 0,
+  kLookupRequest = 1
+};
+
+void HandleEcho(Dart_Port reply_port, Dart_CObject* argument);
+void HandleLookup(Dart_Port port_id, char* address);
+
+#endif