Add support for transformer clusters to barback.

pkg/barback/lib/src/barback.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.barback;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 80 matching lines @@
   void removeSources(Iterable<AssetId> removed) =>
       _graph.removeSources(removed);
 
   /// Gets all output assets.
   ///
   /// If a build is currently in progress, waits until it completes. The
   /// returned future will complete with a [BarbackException] if the build is
   /// not successful.
   Future<AssetSet> getAllAssets() => _graph.getAllAssets();
 
-  /// Sets the transformer phases for [package]'s assets to [transformers].
+  /// Sets the operator phases for [package]'s assets to [operators].
   ///
-  /// To the extent that [transformers] is similar to the previous transformer
-  /// phases for [package], the existing asset graph will be preserved.
-  void updateTransformers(String package,
-          Iterable<Iterable<Transformer>> transformers) =>
-      _graph.updateTransformers(package, transformers);
+  /// To the extent that [operators] is similar to the previous operator phases
+  /// for [package], the existing asset graph will be preserved.
+  void updateOperators(String package,
+          Iterable<Iterable<Operator>> operators) =>
+      _graph.updateOperators(package, operators);
 }