Reorganize barback's source files.

pkg/barback/lib/src/graph/phase_output.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_output;
+library barback.graph.phase_output;
 
 import 'dart:async';
 import 'dart:collection';
 
-import 'asset_forwarder.dart';
-import 'asset_node.dart';
-import 'errors.dart';
+import '../asset/asset_forwarder.dart';
+import '../asset/asset_node.dart';
+import '../errors.dart';
 import 'phase.dart';
 
 /// A class that handles a single output of a phase.
 ///
 /// Normally there's only a single [AssetNode] for a phase's output, but it's
 /// possible that multiple transformers in the same phase emit assets with the
 /// same id, causing collisions. This handles those collisions by forwarding the
 /// chronologically first asset.
 ///
 /// When the asset being forwarding changes, the old value of [output] will be
@@ skipping 82 matching lines @@
       // they've successfully resolved the collision or not.
       if (_assets.length > 1) {
         // TODO(nweiz): report this through the output asset.
         _phase.cascade.reportError(collisionException);
       }
     });
   }
 
   String toString() => "phase output in $_location for $output";
 }