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

pkg/intl/test/message_extraction/foo_messages_all.dart

 /**
  * DO NOT EDIT. This is code generated via pkg/intl/generate_localized.dart
  * This is a library that looks up messages for specific locales by
  * delegating to the appropriate library.
  */
 
 library messages_all;
 
 import 'dart:async';
 import 'package:intl/message_lookup_by_library.dart';
@@ skipping 16 matching lines @@
 Future initializeMessages(String localeName) {
   initializeInternalMessageLookup(() => new CompositeMessageLookup());
   messageLookup.addLocale(localeName, _findGeneratedMessagesFor);
   // TODO(alanknight): Restore once Issue 12824 is fixed.
   // var lib = deferredLibraries[localeName];
   // return lib == null ? new Future.value(false) : lib.load();
   return new Future.value(true);
 }
 
 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);
 }