Reorganize barback's source files.

pkg/barback/lib/src/graph/phase.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;
+library barback.graph.phase;
 
 import 'dart:async';
 
+import '../asset/asset_id.dart';
+import '../asset/asset_node.dart';
+import '../asset/asset_node_set.dart';
+import '../errors.dart';
+import '../log.dart';
+import '../transformer/transformer.dart';
+import '../transformer/transformer_group.dart';
+import '../utils.dart';
+import '../utils/multiset.dart';
 import 'asset_cascade.dart';
-import 'asset_id.dart';
-import 'asset_node.dart';
-import 'asset_node_set.dart';
-import 'errors.dart';
 import 'group_runner.dart';
-import 'log.dart';
-import 'multiset.dart';
 import 'node_status.dart';
 import 'node_streams.dart';
 import 'phase_forwarder.dart';
 import 'phase_output.dart';
-import 'transformer.dart';
 import 'transformer_classifier.dart';
-import 'transformer_group.dart';
-import 'utils.dart';
 
 /// One phase in the ordered series of transformations in an [AssetCascade].
 ///
 /// Each phase can access outputs from previous phases and can in turn pass
 /// outputs to later phases. Phases are processed strictly serially. All
 /// transforms in a phase will be complete before moving on to the next phase.
 /// Within a single phase, all transforms will be run in parallel.
 ///
 /// Building can be interrupted between phases. For example, a source is added
 /// which starts the background process. Sometime during, say, phase 2 (which
@@ skipping 340 matching lines @@
     assert(asset.state.isDirty);
     asset.force();
     asset.whenStateChanges().then((state) {
       if (state.isRemoved) return getOutput(asset.id);
       return asset;
     }).then(request.complete).catchError(request.completeError);
   }
 
   String toString() => "phase $_location.$_index";
 }