Support cross-package transforms in barback.

pkg/barback/lib/src/transform_node.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.transform_node;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 109 matching lines @@
 
       // Don't allow partial results from a failed transform.
       newOutputs.clear();
     }).then((_) {
       if (_isDirty) return [];
 
       return _adjustOutputs(newOutputs);
     });
   }
 
+  /// Gets the asset for an input [id].
+  ///
+  /// If an input with that ID cannot be found, throws an
+  /// [AssetNotFoundException].
   Future<Asset> getInput(AssetId id) {
-    return newFuture(() {
-      var node = phase.inputs[id];
-      // TODO(rnystrom): Need to handle passthrough where an asset from a
-      // previous phase can be found.
-
+    return phase.getInput(id).then((node) {
       // Throw if the input isn't found. This ensures the transformer's apply
       // is exited. We'll then catch this and report it through the proper
       // results stream.
       if (node == null) throw new MissingInputException(id);
 
       // If the asset node is found, wait until its contents are actually
       // available before we return them.
       return node.whenAvailable.then((asset) {
         _inputSubscriptions.putIfAbsent(node.id,
             () => node.onStateChange.listen((_) => _dirty()));
@@ skipping 38 matching lines @@
       } else {
         var controller = new AssetNodeController.available(asset);
         _outputControllers[asset.id] = controller;
         brandNewOutputs.add(controller.node);
       }
     }
 
     return brandNewOutputs;
   }
 }