Add support for transformer clusters to barback.

pkg/barback/test/utils.dart

Index: pkg/barback/test/utils.dart
diff --git a/pkg/barback/test/utils.dart b/pkg/barback/test/utils.dart
index 7fb1a23b0ee4da94378de14c4d6ff163bf4135f7..70fba524a15b8a13b99c68b575650b860f4ec16b 100644
--- a/pkg/barback/test/utils.dart
+++ b/pkg/barback/test/utils.dart
@@ -132,8 +132,7 @@ void removeSourcesSync(Iterable assets) =>
     _barback.removeSources(_parseAssets(assets));
 
 /// Sets the transformers for [package] to [transformers].
-void updateTransformers(String package,
-    Iterable<Iterable<Transformer>> transformers) {
+void updateTransformers(String package, Iterable<Iterable> transformers) {
   schedule(() => _barback.updateTransformers(package, transformers),
       "updating transformers for $package");
 }