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

pkg/intl/lib/generate_localized.dart

Index: pkg/intl/lib/generate_localized.dart
diff --git a/pkg/intl/lib/generate_localized.dart b/pkg/intl/lib/generate_localized.dart
index f17e7c48969ea1c82c84e14ef2fcbfa0ff8ee4b6..b5ccd2824fa25d2cd1ec35ea1b1fffca638894ac 100644
--- a/pkg/intl/lib/generate_localized.dart
+++ b/pkg/intl/lib/generate_localized.dart
@@ -226,7 +226,8 @@ Future initializeMessages(String localeName) {
 }
 
 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);
 }