Reorganize barback's source files.

pkg/barback/lib/src/barback.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.barback;
 
 import 'dart:async';
 
-import 'asset.dart';
-import 'asset_id.dart';
-import 'asset_set.dart';
+import 'asset/asset.dart';
+import 'asset/asset_id.dart';
+import 'asset/asset_set.dart';
 import 'log.dart';
 import 'build_result.dart';
 import 'errors.dart';
-import 'package_graph.dart';
+import 'graph/package_graph.dart';
 import 'package_provider.dart';
-import 'transformer.dart';
+import 'transformer/transformer.dart';
 
 /// A general-purpose asynchronous build dependency graph manager.
 ///
 /// It consumes source assets (including Dart files) in a set of packages,
 /// runs transformations on them, and then tracks which sources have been
 /// modified and which transformations need to be re-run.
 ///
 /// To do this, you give barback a [PackageProvider] which can yield a set of
 /// [Transformer]s and raw source [Asset]s. Then you tell it which input files
 /// have been added or modified by calling [updateSources]. Barback will
@@ skipping 81 matching lines @@
   ///
   /// To the extent that [transformers] is similar to the previous transformer
   /// phases for [package], the existing asset graph will be preserved.
   ///
   /// Elements of the inner iterable of [transformers] must be either
   /// [Transformer]s or [TransformerGroup]s.
   void updateTransformers(String package,
           Iterable<Iterable> transformers) =>
       _graph.updateTransformers(package, transformers);
 }