Call asyncEnd only on success.

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.
-/// This library provides two methods
+/// This library provides four methods
 ///  - asyncStart(): Needs to be called before an asynchronous operation is
 ///                  scheduled.
 ///  - asyncEnd(): Needs to be called as soon as the asynchronous operation
 ///                ended.
+///  - asyncSuccess(_): Variant of asyncEnd useful together with Future.then.
+///  - asyncTest(f()): Helper method that wraps a computation that returns a
+///                    Future with matching calls to asyncStart() and
+///                    asyncSuccess(_).
 /// After the last asyncStart() called was matched with a corresponding
-/// asyncEnd() call, the testing driver will be notified that the tests is done.
+/// asyncEnd() or asyncSuccess(_) 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.');
 }
 
+/// Call this method before an asynchronous test is created.
 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++;
 }
 
+/// Call this after an asynchronous test has ended successfully.
 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');
   }
 }
 
+/**
+ * Call this after an asynchronous test has ended successfully. This is a helper
+ * for calling [asyncEnd].
+ *
+ * This method intentionally has a signature that matches [:Future.then:] as a
+ * convenience for calling [asyncEnd] when a [:Future:] completes without error,
+ * like this:
+ *
+ *     asyncStart();
+ *     Future result = test();
+ *     result.then(asyncSuccess);
+ */
+void asyncSuccess(_) => asyncEnd();
+
+/**
+ * Helper method for performing asynchronous tests involving [:Future:].
+ *
+ * [f] must return a [:Future:] for the test computation.
+ */
 void asyncTest(f()) {
   asyncStart();
-  f().whenComplete(() => asyncEnd());
+  f().then(asyncSuccess);
 }