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

pkg/intl/lib/generate_localized.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 provides utilities for generating localized versions of
  * messages. It does not stand alone, but expects to be given
  * TranslatedMessage objects and generate code for a particular locale
  * based on them.
  *
@@ skipping 208 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);
 }
 """;