Make Transform a little more pleasant to use:

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';
 
@@ skipping 11 matching lines @@
   OneToManyTransformer(this.extension);
 
   Future<bool> doIsPrimary(Asset asset) =>
     new Future.value(asset.id.extension == ".$extension");
 
   Future doApply(Transform transform) {
     return getPrimary(transform)
         .then((input) => input.readAsString())
         .then((lines) {
       for (var line in lines.split(",")) {
-        var id = new AssetId(transform.primaryId.package, line);
+        var id = new AssetId(transform.primaryInput.id.package, line);
         transform.addOutput(new Asset.fromString(id, "spread $extension"));
       }
     });
   }
 
   String toString() => "1->many $extension";
 }