Remove barback from the repo.

pkg/barback/lib/src/asset/asset_node_set.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.asset_node_set;
-
-import 'package:collection/collection.dart';
-
-import 'asset_id.dart';
-import 'asset_node.dart';
-
-/// A set of [AssetNode]s that automatically ensures that nodes are removed from
-/// the set as soon as they're marked as [AssetState.REMOVED].
-///
-/// Asset nodes may be accessed by their ids. This means that only one node with
-/// a given id may be stored in the set at a time.
-class AssetNodeSet extends DelegatingSet<AssetNode> {
-  // TODO(nweiz): Use DelegatingMapSet when issue 18705 is fixed.
-  /// A map from asset ids to assets in the set.
-  final _assetsById = new Map<AssetId, AssetNode>();
-
-  AssetNodeSet()
-      : super(new Set());
-
-  /// Returns the asset node in the set with [id], or `null` if none exists.
-  AssetNode operator [](AssetId id) => _assetsById[id];
-
-  bool add(AssetNode node) {
-    if (node.state.isRemoved) return false;
-    node.whenRemoved(() {
-      super.remove(node);
-      _assetsById.remove(node.id);
-    });
-    _assetsById[node.id] = node;
-    return super.add(node);
-  }
-
-  /// Returns whether an asset node with the given [id] is in the set.
-  bool containsId(AssetId id) => _assetsById.containsKey(id);
-
-  void addAll(Iterable<AssetNode> nodes) => nodes.forEach(add);
-}