Reorganize barback's source files.

pkg/barback/lib/src/graph/asset_cascade.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_cascade;
+library barback.graph.asset_cascade;
 
 import 'dart:async';
 
-import 'asset.dart';
-import 'asset_id.dart';
-import 'asset_node.dart';
-import 'asset_set.dart';
-import 'cancelable_future.dart';
-import 'errors.dart';
-import 'log.dart';
+import '../asset/asset.dart';
+import '../asset/asset_id.dart';
+import '../asset/asset_node.dart';
+import '../asset/asset_set.dart';
+import '../errors.dart';
+import '../log.dart';
+import '../transformer/transformer.dart';
+import '../utils/cancelable_future.dart';
 import 'node_status.dart';
 import 'node_streams.dart';
 import 'package_graph.dart';
 import 'phase.dart';
-import 'transformer.dart';
 
 /// The asset cascade for an individual package.
 ///
 /// This keeps track of which [Transformer]s are applied to which assets, and
 /// re-runs those transformers when their dependencies change. The transformed
 /// asset nodes are accessible via [getAssetNode].
 ///
 /// A cascade consists of one or more [Phases], each of which has one or more
 /// [Transformer]s that run in parallel, potentially on the same inputs. The
 /// inputs of the first phase are the source assets for this cascade's package.
@@ skipping 182 matching lines @@
     _streams.onLogPool.add(phase.onLog);
     if (_phaseStatusSubscription != null) _phaseStatusSubscription.cancel();
     _phaseStatusSubscription =
         phase.onStatusChange.listen(_streams.changeStatus);
 
     _phases.add(phase);
   }
 
   String toString() => "cascade for $package";
 }