Fix some warnings in pub and pkg packages.

pkg/intl/example/basic/messages_th_th.dart

 // Copyright (c) 2012, 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.
 
 /**
  * This is a library that provides messages for a German locale. All the
  * messages from the main program should be duplicated here with the same
  * function name.
  */
 library messages_th_TH;
 import 'package:intl/intl.dart';
 import 'package:intl/message_lookup_by_library.dart';
 
 final messages = new MessageLookup();
 
 class MessageLookup extends MessageLookupByLibrary {
 
   get localeName => 'th_TH';
 
+  bool localeExists(localeName) => null;
+
   final messages = {
     "runAt" : (time, day) => Intl.message('วิ่ง $time on $day.')
   };
 }