Reorganize barback's source files.

pkg/barback/lib/src/graph/phase_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.phase_forwarder;
+library barback.graph.phase_forwarder;
 
 import 'dart:async';
 
-import 'asset_node.dart';
-import 'asset_node_set.dart';
+import '../asset/asset_node.dart';
+import '../asset/asset_node_set.dart';
 
 /// A class that takes care of forwarding assets within a phase.
 ///
 /// Each phase contains one or more channels that process its input assets. Each
 /// non-grouped transformer for that phase is a channel, each [TransformerGroup]
 /// in that phase is another, and the source node is the final channel. For each
 /// input asset, each channel individually decides whether to forward that asset
 /// based on whether that channel uses it. If a channel does decide to forward
 /// an asset, we call that forwarded asset an "intermediate forwarded asset" to
 /// distinguish it from the output of a [PhaseForwarder].
@@ skipping 104 matching lines @@
 
     // If we're already forwarding a final asset, set it dirty iff any of the
     // intermediate assets are dirty.
     if (_intermediateAssets.any((asset) => asset.state.isDirty)) {
       if (!_outputController.node.state.isDirty) _outputController.setDirty();
     } else if (!_outputController.node.state.isAvailable) {
       _outputController.setAvailable(_intermediateAssets.first.asset);
     }
   }
 }