Add support for transformer clusters 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 313d42cae1f9c6b364ac38afac574892a2a4ef93..2393f1f004a27be18f096202a0aaa9bf239eb475 100644
--- a/pkg/barback/lib/src/barback.dart
+++ b/pkg/barback/lib/src/barback.dart
@@ -102,7 +102,10 @@ class Barback {
   ///
   /// 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);
 }