Reorganize barback's source files.

pkg/barback/lib/src/graph/package_graph.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.package_graph;
+library barback.graph.package_graph;
 
 import 'dart:async';
 import 'dart:collection';
 
+import '../asset/asset_id.dart';
+import '../asset/asset_node.dart';
+import '../asset/asset_set.dart';
+import '../build_result.dart';
+import '../errors.dart';
+import '../log.dart';
+import '../package_provider.dart';
+import '../transformer/transformer.dart';
+import '../utils.dart';
 import 'asset_cascade.dart';
-import 'asset_id.dart';
-import 'asset_node.dart';
-import 'asset_set.dart';
-import 'build_result.dart';
-import 'errors.dart';
-import 'log.dart';
 import 'node_status.dart';
-import 'package_provider.dart';
-import 'transformer.dart';
-import 'utils.dart';
 
 /// The collection of [AssetCascade]s for an entire application.
 ///
 /// This tracks each package's [AssetCascade] and routes asset requests between
 /// them.
 class PackageGraph {
   /// The provider that exposes asset and package information.
   final PackageProvider provider;
 
   /// The [AssetCascade] for each package.
@@ skipping 224 matching lines @@
         completer.completeError(error, stackTrace);
       });
     }, onError: (error, stackTrace) {
       _lastUnexpectedError = error;
       _lastUnexpectedErrorTrace = stackTrace;
       _resultsController.addError(error, stackTrace);
     });
     return completer.future;
   }
 }