Fix cancelable_future again.

pkg/barback/test/transformer/many_to_one.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.many_to_one;
+
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:barback/src/utils.dart';
 
 import 'mock.dart';
 
 /// A transformer that uses the contents of a file to define the other inputs.
 ///
 /// Outputs a file with the same name as the primary but with an "out"
@@ skipping 20 matching lines @@
         return getInput(transform, id).then((input) => input.readAsString());
       }));
     }).then((outputs) {
       var id = transform.primaryId.changeExtension(".out");
       transform.addOutput(new Asset.fromString(id, outputs.join()));
     });
   }
 
   String toString() => "many->1 $extension";
 }