Add the ability to dynamically modify transforms 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 70 matching lines @@
   ///
   /// Begins applying any transforms that can consume any of the sources. If a
   /// given source is already known, it is considered modified and all
   /// transforms that use it will be re-applied.
   void updateSources(Iterable<AssetId> sources) =>
       _graph.updateSources(sources);
 
   /// Removes [removed] from the graph's known set of source assets.
   void removeSources(Iterable<AssetId> removed) =>
       _graph.removeSources(removed);
-}
+
+  /// 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.
+  void updateTransformers(String package,
+          Iterable<Iterable<Transformer>> transformers) =>
+      _graph.updateTransformers(package, transformers);
+}