Update yield warnings.

pkg/compiler/lib/src/warnings.dart

 // Copyright (c) 2011, 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.
 
 part of dart2js;
 
 const DONT_KNOW_HOW_TO_FIX = "Computer says no!";
 
 /**
  * The messages in this file should meet the following guide lines:
@@ skipping 2067 matching lines @@
   static const MessageKind EXPERIMENTAL_ASYNC_AWAIT = const MessageKind(
       "Experimental language feature 'async/await' is not supported.");
 
   static const MessageKind INVALID_STARRED_KEYWORD = const MessageKind(
       "Invalid '#{keyword}' keyword.",
       options: const ['--enable-async'],
       howToFix: "Try removing whitespace between '#{keyword}' and '*'.",
       examples: const [
         "main() async * {}",
         "main() sync * {}",
-        "main() async* { yield * 0; }"
+        "main() async* { yield * null; }"
       ]);
 
   static const MessageKind INVALID_SYNC_MODIFIER = const MessageKind(
       "Invalid modifier 'sync'.",
       options: const ['--enable-async'],
       howToFix: "Try replacing 'sync' with 'sync*'.",
       examples: const [
         "main() sync {}"
       ]);
 
@@ skipping 340 matching lines @@
   static String convertToString(value) {
     if (value is ErrorToken) {
       // Shouldn't happen.
       return value.assertionMessage;
     } else if (value is Token) {
       value = value.value;
     }
     return '$value';
   }
 }