Write a README for the Intl package

pkg/intl/test/date_time_format_test_stub.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.
 
 /**
  * Test date formatting and parsing using locale data which is available
  * directly in the program as a constant.
  */
 
 library date_time_format_test;
@@ skipping 35 matching lines @@
 void runWith(TestListFunc getSubset, String dir,
              InitializeDateFormattingFunc initFunction) {
   // Initialize one locale just so we know what the list is.
   // Also, note that we take the list of locales as a function so that we don't
   // evaluate it until after we know that all the locales are available.
 
   bool initialized = false;
 
   setUp(() {
     if(initialized) {
-      return;
+      return null;
     }
     return initFunction("en_US", dir)
         .then((_) {
           return Future.forEach(DateFormat.allLocalesWithSymbols(), (locale) {
             return initFunction(locale, dir);
           });
         })
         .then((_) {
           initialized = true;
         });
   });
 
   runDateTests(getSubset);
 }