Fix cancelable_future again.

pkg/barback/test/transformer/rewrite.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.rewrite;
+
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 
 import 'mock.dart';
 
 /// A [Transformer] that takes assets ending with one extension and generates
 /// assets with a given extension.
 ///
 /// Appends the output extension to the contents of the input file.
@@ skipping 17 matching lines @@
         var id = transform.primaryId.changeExtension(".$extension");
         return input.readAsString().then((content) {
           transform.addOutput(new Asset.fromString(id, "$content.$extension"));
         });
       }));
     });
   }
 
   String toString() => "$from->$to";
 }