Add support for transformer clusters to barback.

pkg/barback/lib/src/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.asset_cascade;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset.dart';
@@ skipping 81 matching lines @@
   /// A future that completes when the currently running build process finishes.
   ///
   /// If no build it in progress, is `null`.
   Future _processDone;
 
   /// Whether any source assets have been updated or removed since processing
   /// last began.
   var _newChanges = false;
 
   /// Returns all currently-available output assets from this cascade.
-  AssetSet get availableOutputs => _phases.last.availableOutputs;
+  AssetSet get availableOutputs =>
+    new AssetSet.from(_phases.last.availableOutputs.map((node) => node.asset));
 
   /// Creates a new [AssetCascade].
   ///
   /// It loads source assets within [package] using [provider].
   AssetCascade(this.graph, this.package) {
     _onDirtyPool.add(_onDirtyController.stream);
     _addPhase(new Phase(this, []));
   }
 
   /// Gets the asset identified by [id].
@@ skipping 70 matching lines @@
       // If the source was being loaded, cancel that load.
       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].
-  void updateTransformers(Iterable<Iterable<Transformer>> transformers) {
+  ///
+  /// Elements of the inner iterable of [transformers] must be either
+  /// [Transformer]s or [TransformerGroup]s.
+  void updateTransformers(Iterable<Iterable> transformers) {
     transformers = transformers.toList();
 
     for (var i = 0; i < transformers.length; i++) {
       if (_phases.length > i) {
         _phases[i].updateTransformers(transformers[i]);
         continue;
       }
 
       _addPhase(_phases.last.addPhase(transformers[i]));
     }
@@ skipping 72 matching lines @@
 
         // Otherwise, everything is done.
         return;
       }
 
       // Process that phase and then loop onto the next.
       return future.then((_) => _process());
     });
   }
 }