Fix cancelable_future again.

pkg/barback/test/transformer/create_asset.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.create_asset;
+
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:barback/src/utils.dart';
 
 import 'mock.dart';
 
 /// A transformer that outputs an asset with the given id.
 class CreateAssetTransformer extends MockTransformer {
   final String output;
 
   CreateAssetTransformer(this.output);
 
   Future<bool> doIsPrimary(Asset asset) => new Future.value(true);
 
   Future doApply(Transform transform) {
     return newFuture(() {
       transform.addOutput(
           new Asset.fromString(new AssetId.parse(output), output));
     });
   }
 }