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

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 95163724ea228c92071b2ab5140c5f631fdde6a0..647702378b8eb032fe78b93ce64f9c5a423ebdeb 100644
--- a/pkg/intl/lib/generate_localized.dart
+++ b/pkg/intl/lib/generate_localized.dart
@@ -215,7 +215,8 @@ const closing = """
 Future initializeMessages(String localeName) {
   initializeInternalMessageLookup(() => new CompositeMessageLookup());
   messageLookup.addLocale(localeName, _findGeneratedMessagesFor);
-  return deferredLibraries[localeName].load();
+  var lib = deferredLibraries[localeName];                                                           
+  return lib == null ? new Future.value(false) : lib.load();     
 }
 
 MessageLookupByLibrary _findGeneratedMessagesFor(locale) {