Reorganize barback's source files.

pkg/barback/lib/src/transformer/declaring_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.declaring_transform;
+library barback.transformer.declaring_transform;
 
-import 'asset_id.dart';
+import '../asset/asset_id.dart';
+import '../graph/transform_node.dart';
 import 'base_transform.dart';
-import 'transform_node.dart';
 
 /// A transform for [DeclaringTransform]ers that allows them to declare the ids
 /// of the outputs they'll generate without generating the concrete bodies of
 /// those outputs.
 class DeclaringTransform extends BaseTransform {
   final _outputIds = new Set<AssetId>();
 
   final AssetId primaryId;
 
   DeclaringTransform._(TransformNode node)
@@ skipping 18 matching lines @@
 class DeclaringTransformController extends BaseTransformController {
   DeclaringTransform get transform => super.transform;
 
   /// The set of ids that the transformer declares it will emit for the given
   /// primary input.
   Set<AssetId> get outputIds => transform._outputIds;
 
   DeclaringTransformController(TransformNode node)
       : super(new DeclaringTransform._(node));
 }