Reorganize barback's source files.

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_node_set;
+library barback.asset.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);
 }