Reorganize barback's source files.

pkg/barback/lib/src/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_node_set;
-
-import 'package:collection/collection.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].
-class AssetNodeSet extends DelegatingSet<AssetNode> {
-  AssetNodeSet()
-      : super(new Set());
-
-  bool add(AssetNode node) {
-    if (node.state.isRemoved) return false;
-    node.whenRemoved(() => super.remove(node));
-    return super.add(node);
-  }
-
-  void addAll(Iterable<AssetNode> nodes) => nodes.forEach(add);
-}