Rationalize Dart mojo and sky package structure

mojo/dart/testing/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.
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 /**
  * Helper method for performing asynchronous tests involving [:Future:].
  *
  * [f] must return a [:Future:] for the test computation.
  */
 void asyncTest(f()) {
   asyncStart();
   f().then(asyncSuccess);
 }