Add async_helper package.

pkg/async_helper/lib/async_helper.dart

 // Copyright (c) 2013, 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 library is used for testing asynchronous tests.
 /// If a test is asynchronous, it needs to notify the testing driver
 /// about this (otherwise tests may get reported as passing [after main()
 /// finished] even if the asynchronous operations fail).
 /// Tests which can't use the unittest framework should use the helper functions
 /// in this library.
@@ skipping 43 matching lines @@
                             'asyncStart().');
     }
   }
   _asyncLevel--;
   if (_asyncLevel == 0) {
     _port.close();
     _port = null;
     print('unittest-suite-success');
   }
 }