pkg/barback: removed unused imports, added a missing import

pkg/barback/lib/src/asset_cascade.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.asset_cascade;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
 import 'build_result.dart';
 import 'cancelable_future.dart';
 import 'errors.dart';
-import 'change_batch.dart';
 import 'package_graph.dart';
 import 'phase.dart';
 import 'transformer.dart';
 import 'utils.dart';
 
 /// The asset cascade for an individual package.
 ///
 /// This keeps track of which [Transformer]s are applied to which assets, and
 /// re-runs those transformers when their dependencies change. The transformed
 /// asset nodes are accessible via [getAssetNode].
@@ skipping 219 matching lines @@
 
         // Otherwise, everything is done.
         return;
       }
 
       // Process that phase and then loop onto the next.
       return future.then((_) => _process());
     });
   }
 }