Reorganize barback's source files.

pkg/barback/lib/src/graph/phase.dart

Index: pkg/barback/lib/src/graph/phase.dart
diff --git a/pkg/barback/lib/src/phase.dart b/pkg/barback/lib/src/graph/phase.dart
similarity index 97%
rename from pkg/barback/lib/src/phase.dart
rename to pkg/barback/lib/src/graph/phase.dart
index b943141fd2ebea1b698bba91bd4bd079c50953e5..70f7e7fe354c8c7ce03b67114111363863163c7e 100644
--- a/pkg/barback/lib/src/phase.dart
+++ b/pkg/barback/lib/src/graph/phase.dart
@@ -2,26 +2,26 @@
 // 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].
 ///