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..1a1ffc97befcc280a0b5107aa56a5760ee8d1501 100644
--- a/pkg/barback/lib/src/barback.dart
+++ b/pkg/barback/lib/src/barback.dart
@@ -98,11 +98,11 @@ class Barback {
   /// not successful.
   Future<AssetSet> getAllAssets() => _graph.getAllAssets();
 
-  /// Sets the transformer phases for [package]'s assets to [transformers].
+  /// Sets the operator phases for [package]'s assets to [operators].
   ///
-  /// 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);
+  /// To the extent that [operators] is similar to the previous operator phases
+  /// for [package], the existing asset graph will be preserved.
+  void updateOperators(String package,
+          Iterable<Iterable<Operator>> operators) =>
+      _graph.updateOperators(package, operators);
 }