Reorganize barback's source files.

pkg/barback/lib/src/graph/transformer_classifier.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.transformer_classifier;
+library barback.graph.transformer_classifier;
 
 import 'dart:async';
 
-import 'asset_forwarder.dart';
-import 'asset_node.dart';
-import 'errors.dart';
-import 'log.dart';
+import '../asset/asset_forwarder.dart';
+import '../asset/asset_node.dart';
+import '../errors.dart';
+import '../log.dart';
+import '../transformer/transformer.dart';
+import '../utils.dart';
 import 'node_status.dart';
 import 'node_streams.dart';
 import 'phase.dart';
 import 'transform_node.dart';
-import 'transformer.dart';
-import 'utils.dart';
 
 /// A class for classifying the primary inputs for a transformer according to
 /// its `classifyPrimary` method.
 ///
 /// This is used for non-aggregate transformers; they're modeled as aggregate
 /// transformers that return the primary path if `isPrimary` is true and `null`
 /// if `isPrimary` is `null`.
 class TransformerClassifier {
   /// The containing [Phase].
   final Phase _phase;
@@ skipping 89 matching lines @@
 
   /// Force all deferred transforms to begin producing concrete assets.
   void forceAllTransforms() {
     for (var transform in _transforms.values) {
       transform.force();
     }
   }
 
   String toString() => "classifier in $_location for $transformer";
 }