Make Transform a little more pleasant to use:

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 '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"
 /// extension containing the concatenated contents of all non-primary inputs.
 class ManyToOneTransformer extends MockTransformer {
   final String extension;
 
   /// Creates a transformer that consumes assets with [extension].
   ///
   /// That file contains a comma-separated list of paths and it will input
   /// files at each of those paths.
   ManyToOneTransformer(this.extension);
 
   Future<bool> doIsPrimary(Asset asset) =>
     new Future.value(asset.id.extension == ".$extension");
 
   Future doApply(Transform transform) {
-    return getPrimary(transform)
-        .then((primary) => primary.readAsString())
-        .then((contents) {
+    return transform.primaryInput.readAsString().then((contents) {
       // Get all of the included inputs.
       return Future.wait(contents.split(",").map((path) {
         var id;
         if (path.contains("|")) {
           id = new AssetId.parse(path);
         } else {
-          id = new AssetId(transform.primaryId.package, path);
+          id = new AssetId(transform.primaryInput.id.package, path);
         }
         return getInput(transform, id).then((input) => input.readAsString());
       }));
     }).then((outputs) {
-      var id = transform.primaryId.changeExtension(".out");
+      var id = transform.primaryInput.id.changeExtension(".out");
       transform.addOutput(new Asset.fromString(id, outputs.join()));
     });
   }
 
   String toString() => "many->1 $extension";
 }