Reorganize barback's source files.

pkg/barback/lib/src/transformer/transform.dart

 // Copyright (c) 2014, 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.transform;
+library barback.transformer.transform;
 
 import 'dart:async';
 import 'dart:convert';
 
-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 '../errors.dart';
+import '../graph/transform_node.dart';
+import '../utils.dart';
 import 'base_transform.dart';
-import 'errors.dart';
-import 'transform_node.dart';
-import 'utils.dart';
 
 /// While a [Transformer] represents a *kind* of transformation, this defines
 /// one specific usage of it on a set of files.
 ///
 /// This ephemeral object exists only during an actual transform application to
 /// facilitate communication between the [Transformer] and the code hosting
 /// the transformation. It lets the [Transformer] access inputs and generate
 /// outputs.
 class Transform extends BaseTransform {
   final TransformNode _node;
@@ skipping 83 matching lines @@
 /// The controller for [Transform].
 class TransformController extends BaseTransformController {
   Transform get transform => super.transform;
 
   /// The set of assets that the transformer has emitted.
   AssetSet get outputs => transform._outputs;
 
   TransformController(TransformNode node)
       : super(new Transform._(node));
 }