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

pkg/intl/test/message_extraction/sample_with_messages.dart

Index: pkg/intl/test/message_extraction/sample_with_messages.dart
diff --git a/pkg/intl/test/message_extraction/sample_with_messages.dart b/pkg/intl/test/message_extraction/sample_with_messages.dart
index 292273c4d0bcaa406e33a1be7da88afe273b845d..753f39c23f27447d9b717475c5858e29f1dea6f8 100644
--- a/pkg/intl/test/message_extraction/sample_with_messages.dart
+++ b/pkg/intl/test/message_extraction/sample_with_messages.dart
@@ -178,6 +178,8 @@ main() {
   var fr = new Intl("fr");
   var english = new Intl("en_US");
   var de = new Intl("de_DE");
+  // Throw in an initialize of a null locale to make sure it doesn't throw.
+  initializeMessages(null);
   initializeMessages(fr.locale).then((_) => printStuff(fr));
   initializeMessages(de.locale).then((_) => printStuff(de));
   printStuff(english);