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

pkg/barback/lib/src/change_batch.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.change_batch;
 
-import 'asset.dart';
 import 'asset_id.dart';
 
 /// Represents a batch of source asset changes: additions, removals and
 /// modifications.
 class ChangeBatch {
   /// The assets that have been added or modified in this batch.
   final updated = new Set<AssetId>();
 
   /// The assets that have been removed in this batch.
   final removed = new Set<AssetId>();
 
   /// Adds the updated [assets] to this batch.
   void update(Iterable<AssetId> assets) {
     updated.addAll(assets);
 
     // If they were previously removed, they are back now.
     removed.removeAll(assets);
   }
 
   /// Removes [assets] from this batch.
   void remove(Iterable<AssetId> assets) {
     removed.addAll(assets);
 
     // If they were previously updated, they are gone now.
     updated.removeAll(assets);
   }
 }