Make Transform a little more pleasant to use:

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';
 
@@ skipping 13 matching lines @@
   /// [to] may be a space-separated list in which case multiple outputs will be
   /// created for each input.
   RewriteTransformer(this.from, this.to);
 
   Future<bool> doIsPrimary(Asset asset) =>
     new Future.value(asset.id.extension == ".$from");
 
   Future doApply(Transform transform) {
     return getPrimary(transform).then((input) {
       return Future.wait(to.split(" ").map((extension) {
-        var id = transform.primaryId.changeExtension(".$extension");
+        var id = input.id.changeExtension(".$extension");
         return input.readAsString().then((content) {
           transform.addOutput(new Asset.fromString(id, "$content.$extension"));
         });
       }));
     });
   }
 
   String toString() => "$from->$to";
 }