Migrate block 39 language/async_backwards ... /async_star_take.

tests/language_2/async_star_no_cancel_test.dart

 // Copyright (c) 2016, 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.
 
 import "dart:async";
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
 var events = [];
 
@@ skipping 29 matching lines @@
     events.add("invoke cancel");
     subscription.cancel().then((_) => cancelFinished = true);
   });
 
   new Timer(const Duration(milliseconds: 100), () {
     Expect.isFalse(cancelFinished);
     Expect.listEquals(["main", "listen", "invoke cancel"], events);
     asyncEnd();
   });
 }