Abstract is now required on abstract classes so adding that.

pkg/unittest/unittest.dart

 // Copyright (c) 2011, 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.
 
 /**
  * A library for writing dart unit tests.
  *
  * To import this library, specify the relative path to
  * pkg/unittest/unittest.dart.
  *
@@ skipping 798 matching lines @@
   _config.onDone(testsPassed_, testsFailed_, testsErrors_, _tests,
       _uncaughtErrorMessage);
   _initialized = false;
 }
 
 String _fullSpec(String spec) {
   if (spec === null) return '$_currentGroup';
   return _currentGroup != '' ? '$_currentGroup$groupSep$spec' : spec;
 }
 
-void _fail(String message) {
+void fail(String message) {
   throw new ExpectException(message);
 }
 
 /**
  * Lazily initializes the test library if not already initialized.
  */
 ensureInitialized() {
   if (_initialized) {
     return;
   }
@@ skipping 41 matching lines @@
 }
 
 /** Enable a test by ID. */
 void enableTest(int testId) => _setTestEnabledState(testId, true);
 
 /** Disable a test by ID. */
 void disableTest(int testId) => _setTestEnabledState(testId, false);
 
 /** Signature for a test function. */
 typedef void TestFunction();