Verify the default locale in case we're given an invalid one and need to fall back.

pkg/intl/example/basic/messages_all.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 imports all of the different message libraries and provides an
  * [initializeMessages] function that sets up the lookup for a particular
  * library.
  */
 library messages_all;
@@ skipping 14 matching lines @@
   }
 }
 
 initializeMessages(localeName) {
   initializeInternalMessageLookup(() => new CompositeMessageLookup());
   messageLookup.addLocale(localeName, _findGeneratedMessagesFor);
   return new Future.value();
 }
 
 MessageLookupByLibrary _findGeneratedMessagesFor(locale) {
-  var actualLocale = Intl.verifiedLocale(locale, (x) => _findExact(x) != null);
+  var actualLocale = Intl.verifiedLocale(locale, (x) => _findExact(x) != null,
+      onFailure: (_) => null);
   if (actualLocale == null) return null;
   return _findExact(actualLocale);
 }