Fix warning and hint int test

pkg/mdns/lib/src/packet.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.packet;
 
 import 'dart:convert';
 import 'dart:io';
 import 'dart:typed_data';
 
@@ skipping 64 matching lines @@
   if (length < headerSize) return null;
 
   Uint8List data =
       packet is Uint8List ? packet : new Uint8List.fromList(packet);
   ByteData bd = new ByteData.view(data.buffer);
   // Query identifier.
   int id = bd.getUint16(idOffset);
   // Flags.
   int flags = bd.getUint16(flagsOffset);
 
-  if (flags != responseFlags) return;
+  if (flags != responseFlags) return null;
   // Query count.
   int qdcount = bd.getUint16(qdcountOffset);
   // Number of answers.
   int ancount = bd.getUint16(ancountOffset);
   // Number of name server records.
   int nscount = bd.getUint16(nscountOffset);
   // Number of resource records.
   int arcount = bd.getUint16(arcountOffset);
   int offset = headerSize;
 
@@ skipping 82 matching lines @@
   return result;
 }
 
 /// Exceptions thrown by decoder when the packet is invalid.
 class MDnsDecodeException implements Exception {
   /// Exception message.
   final int offset;
   const MDnsDecodeException(this.offset);
   String toString() => 'Decoding error at $offset';
 }