Added async_helper and fixed broken asynchronous test.

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;
+
+
+bool _initialized = false;
+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;
+  }
+  _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) {
+    print('unittest-suite-done');
+  }
+}