Migrate language/async_backwards... ... language/async_star_take...

tests/language_2/async_star_cancel_while_paused_test.dart

 // Copyright (c) 2015, 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.
 
 // This is a regression test for issue 22853.
 
 import "dart:async";
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
@@ skipping 48 matching lines @@
   }
 
   // Release whoever is currently waiting.
   void release([v]) {
     if (_completer != null) {
       _completer.complete(v);
       _completer = null;
     }
   }
 }