Add async initialization for Intl.message and an example

pkg/intl/example/basic/messages_all.dart

+// Copyright (c) 2012, 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.
+
+/**
+ * In order to avoid needing to import the libraries for each locale
+ * individually in each program file, we make a separate library that imports
+ * all of them. In this example there's only one program file, so it doesn't
+ * make very much difference.
+ */
+#library('messages_all.dart');
+
+#import('messages_th_th.dart', prefix: 'th_TH');
+#import('messages_de_de.dart', prefix: 'de_DE');