Fix cancelable_future again.

pkg/barback/test/transformer/mock.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.
 
+library barback.test.transformer.mock;
+
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:barback/src/utils.dart';
 
 /// The abstract base class for transformers used to test barback.
 ///
 /// This adds the ability to pause and resume different components of the
 /// transformers, and to tell whether they're running, when they start running,
 /// and how many times they've run.
@@ skipping 108 matching lines @@
       if (_runningTransforms == 0) _started = new Completer();
     });
   }
 
   /// The wrapped version of [isPrimary] for subclasses to override.
   Future<bool> doIsPrimary(Asset asset);
 
   /// The wrapped version of [doApply] for subclasses to override.
   Future doApply(Transform transform);
 }