Reorganize barback's source files.

pkg/barback/lib/src/asset/asset_forwarder.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_forwarder;
+library barback.asset.asset_forwarder;
 
 import 'dart:async';
 
 import 'asset_node.dart';
 
 /// A wrapper for an [AssetNode] that forwards events to a new node.
 ///
 /// A forwarder is used when a class wants to forward an [AssetNode] that it
 /// gets as an input, but also wants to have control over when that node is
 /// marked as removed. The forwarder can be closed, thus removing its output
@@ skipping 24 matching lines @@
     });
   }
 
   /// Closes the forwarder and marks [node] as removed.
   void close() {
     if (_controller.node.state.isRemoved) return;
     _subscription.cancel();
     _controller.setRemoved();
   }
 }