Expose aggregate transformers in barback.

pkg/barback/lib/src/graph/asset_cascade.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.asset_cascade;
 
 import 'dart:async';
 
 import '../asset/asset.dart';
 import '../asset/asset_id.dart';
@@ skipping 149 matching lines @@
       if (_loadingSources.containsKey(id)) _loadingSources.remove(id).cancel();
 
       var controller = _sourceControllerMap.remove(id);
       // Don't choke if an id is double-removed for some reason.
       if (controller != null) controller.setRemoved();
     });
   }
 
   /// Sets this cascade's transformer phases to [transformers].
   ///
-  /// Elements of the inner iterable of [transformers] must be either
-  /// [Transformer]s or [TransformerGroup]s.
+  /// Elements of the inner iterable of [transformers] must be [Transformer]s,
+  /// [TransformerGroup]s, or [AggregateTransformer]s.
   void updateTransformers(Iterable<Iterable> transformersIterable) {
     var transformers = transformersIterable.toList();
 
     // Always preserve a single phase with no transformers at the beginning of
     // the cascade so that [TransformNode]s in the first populated phase will
     // have something to request assets from.
     for (var i = 0; i < transformers.length; i++) {
       if (_phases.length > i + 1) {
         _phases[i + 1].updateTransformers(transformers[i]);
         continue;
@@ skipping 31 matching lines @@
     _streams.onLogPool.add(phase.onLog);
     if (_phaseStatusSubscription != null) _phaseStatusSubscription.cancel();
     _phaseStatusSubscription =
         phase.onStatusChange.listen(_streams.changeStatus);
 
     _phases.add(phase);
   }
 
   String toString() => "cascade for $package";
 }