Refactor the Transform classes.

pkg/barback/lib/src/phase_output.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_output;
 
 import 'dart:async';
 import 'dart:collection';
 
-import 'asset_cascade.dart';
 import 'asset_forwarder.dart';
 import 'asset_node.dart';
 import 'errors.dart';
 import 'phase.dart';
 import 'utils.dart';
 
 /// A class that handles a single output of a phase.
 ///
 /// Normally there's only a single [AssetNode] for a phase's output, but it's
 /// possible that multiple transformers in the same phase emit assets with the
@@ skipping 87 matching lines @@
         // Pump the event queue to ensure that the removal of the input triggers
         // a new build to which we can attach the error.
         // TODO(nweiz): report this through the output asset.
         newFuture(() => _phase.cascade.reportError(collisionException));
       }
     });
   }
 
   String toString() => "phase output in $_location for $output";
 }