Switch to async_helper package.

tests/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.
-/// This library provides two methods
-///  - asyncStart(): Needs to be called before an asynchronous operation is
-///                  scheduled.
-///  - asyncEnd(): Needs to be called as soon as the asynchronous operation
-///                ended.
-/// After the last asyncStart() called was matched with a corresponding
-/// asyncEnd() call, the testing driver will be notified that the tests is done.
-
-library async_helper;
-
-// TODO(kustermann): This is problematic because we rely on a working
-// 'dart:isolate' (i.e. it is in particular problematic with dart2js).
-// It would be nice if we could use a different mechanism for different
-// runtimes.
-import 'dart:isolate';
-
-bool _initialized = false;
-ReceivePort _port = null;
-int _asyncLevel = 0;
-
-Exception _buildException(String msg) {
-  return new Exception('Fatal: $msg. This is most likely a bug in your test.');
-}
-
-void asyncStart() {
-  if (_initialized && _asyncLevel == 0) {
-    throw _buildException('asyncStart() was called even though we are done '
-                          'with testing.');
-  }
-  if (!_initialized) {
-    print('unittest-suite-wait-for-done');
-    _initialized = true;
-    _port = new ReceivePort();
-  }
-  _asyncLevel++;
-}
-
-void asyncEnd() {
-  if (_asyncLevel <= 0) {
-    if (!_initialized) {
-      throw _buildException('asyncEnd() was called before asyncStart().');
-    } else {
-      throw _buildException('asyncEnd() was called more often than '
-                            'asyncStart().');
-    }
-  }
-  _asyncLevel--;
-  if (_asyncLevel == 0) {
-    _port.close();
-    _port = null;
-    print('unittest-suite-success');
-  }
-}