Add a null check to avoid trying to load libraries that don't exist.

pkg/intl/test/message_extraction/sample_with_messages.dart

 // Copyright (c) 2013, 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 program with various [Intl.message] messages. It just prints
  * all of them, and is used for testing of message extraction, translation,
  * and code generation.
  */
 library sample;
@@ skipping 160 matching lines @@
     print(nestedSelect("CDN", 2));
   });
 }
 
 var localeToUse = 'en_US';
 
 main() {
   var fr = new Intl("fr");
   var english = new Intl("en_US");
   var de = new Intl("de_DE");
+  // Throw in an initialize of a null locale to make sure it doesn't throw.
+  initializeMessages(null);
   initializeMessages(fr.locale).then((_) => printStuff(fr));
   initializeMessages(de.locale).then((_) => printStuff(de));
   printStuff(english);
 }