Support cross-package transforms in barback.

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';
@@ skipping 15 matching lines @@
 
   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) {
       // Get all of the included inputs.
       return Future.wait(contents.split(",").map((path) {
-        var id = new AssetId(transform.primaryId.package, path);
+        var id;
+        if (path.contains("|")) {
+          id = new AssetId.parse(path);
+        } else {
+          id = new AssetId(transform.primaryId.package, path);
+        }
         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";
 }