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 84 matching lines @@
   ///
   /// 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].
   ///
   /// To the extent that [transformers] is similar to the previous transformer
   /// phases for [package], the existing asset graph will be preserved.
+  ///
+  /// Elements of the inner iterable of [transformers] must be either
+  /// [Transformer]s or [TransformerGroup]s.
   void updateTransformers(String package,
-          Iterable<Iterable<Transformer>> transformers) =>
+          Iterable<Iterable> transformers) =>
       _graph.updateTransformers(package, transformers);
 }