Transition barback's infrastructure to an aggregate-based model.

pkg/barback/lib/src/transformer/transformer.dart

Index: pkg/barback/lib/src/transformer/transformer.dart
diff --git a/pkg/barback/lib/src/transformer/transformer.dart b/pkg/barback/lib/src/transformer/transformer.dart
index efacf761305704ea9c3286f43a71b7c38f750e94..b40d61de37c07c2d78925e5ecdccf80df6cb6905 100644
--- a/pkg/barback/lib/src/transformer/transformer.dart
+++ b/pkg/barback/lib/src/transformer/transformer.dart
@@ -73,7 +73,7 @@ abstract class Transformer {
     return false;
   }
 
-  /// Run this transformer on on the primary input specified by [transform].
+  /// Run this transformer on the primary input specified by [transform].
   ///
   /// The [transform] is used by the [Transformer] for two purposes (in
   /// addition to accessing the primary input). It can call `getInput()` to