Add support for transformer clusters to barback.

pkg/barback/lib/src/package_graph.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.package_graph;
 
 import 'dart:async';
 
 import 'asset_cascade.dart';
 import 'asset_id.dart';
@@ skipping 142 matching lines @@
 
   /// Removes [removed] from the graph's known set of source assets.
   void removeSources(Iterable<AssetId> sources) {
     groupBy(sources, (id) => id.package).forEach((package, ids) {
       var cascade = _cascades[package];
       if (cascade == null) throw new ArgumentError("Unknown package $package.");
       cascade.removeSources(ids);
     });
   }
 
-  void updateTransformers(String package,
-      Iterable<Iterable<Transformer>> transformers) {
-    _cascades[package].updateTransformers(transformers);
+  void updateOperators(String package,
+      Iterable<Iterable<Transformer>> operators) {
+    _cascades[package].updateOperators(operators);
   }
 }