Add the ability to dynamically modify transforms to barback.

pkg/barback/lib/src/barback.dart

Index: pkg/barback/lib/src/barback.dart
diff --git a/pkg/barback/lib/src/barback.dart b/pkg/barback/lib/src/barback.dart
index 3dcb23e66d166996a44b27bd55c3606041e794aa..c66ee40abb3fedf7ca1c5736c96ad6410e2d8f41 100644
--- a/pkg/barback/lib/src/barback.dart
+++ b/pkg/barback/lib/src/barback.dart
@@ -88,4 +88,12 @@ class Barback {
   /// 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);
+}