Factor out an input-handling class from Phase 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 114 matching lines @@
         error = new TransformerException(info, error);
       }
 
       // Catch all transformer errors and pipe them to the results stream.
       // This is so a broken transformer doesn't take down the whole graph.
       phase.cascade.reportError(error);
 
       // Don't allow partial results from a failed transform.
       newOutputs.clear();
     }).then((_) {
-      if (_isDirty) return [];
+      if (_isDirty) return new Set();
 
       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) {
@@ skipping 48 matching lines @@
       } else {
         var controller = new AssetNodeController.available(asset, this);
         _outputControllers[asset.id] = controller;
         brandNewOutputs.add(controller.node);
       }
     }
 
     return brandNewOutputs;
   }
 }