Get rid of an unused library in barback.

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_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);
-  }
-}