Fix cancelable_future again.

pkg/barback/test/transformer/one_to_many.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.one_to_many;
+
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 
 import 'mock.dart';
 
 /// A [Transformer] that takes an input asset that contains a comma-separated
 /// list of paths and outputs a file for each path.
 class OneToManyTransformer extends MockTransformer {
   final String extension;
@@ skipping 13 matching lines @@
         .then((lines) {
       for (var line in lines.split(",")) {
         var id = new AssetId(transform.primaryId.package, line);
         transform.addOutput(new Asset.fromString(id, "spread $extension"));
       }
     });
   }
 
   String toString() => "1->many $extension";
 }