Expose aggregate transformers in barback.

pkg/barback/lib/src/graph/group_runner.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.graph.group_runner;
 
 import 'dart:async';
 
 import '../asset/asset_node.dart';
 import '../log.dart';
 import '../transformer/transformer_group.dart';
 import '../utils/stream_pool.dart';
-import 'asset_cascade.dart';
 import 'node_status.dart';
 import 'phase.dart';
 
 /// A class that processes all of the phases in a single transformer group.
 ///
 /// A group takes many inputs, processes them, and emits many outputs.
 class GroupRunner {
   /// The group this runner runs.
   final TransformerGroup _group;
 
@@ skipping 57 matching lines @@
   /// Removes this group and all sub-phases within it.
   void remove() {
     _onLogPool.close();
     for (var phase in _phases) {
       phase.remove();
     }
   }
 
   String toString() => "group in phase $_location for $_group";
 }