Refactor the Transform classes.

pkg/barback/lib/src/phase_input.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.phase_input;
 
 import 'dart:async';
-import 'dart:collection';
 
 import 'asset.dart';
 import 'asset_forwarder.dart';
 import 'asset_node.dart';
 import 'asset_node_set.dart';
 import 'errors.dart';
 import 'log.dart';
 import 'phase.dart';
 import 'stream_pool.dart';
 import 'transform_node.dart';
@@ skipping 309 matching lines @@
     } else if (_passThroughController == null) {
       _passThroughController = new AssetNodeController.from(input);
       _newPassThrough = true;
     } else if (_passThroughController.node.state.isDirty) {
       _passThroughController.setAvailable(input.asset);
     }
   }
 
   String toString() => "phase input in $_location for $input";
 }