Refactor the Transform classes.

pkg/barback/lib/src/transform.dart

Index: pkg/barback/lib/src/transform.dart
diff --git a/pkg/barback/lib/src/transform.dart b/pkg/barback/lib/src/transform.dart
index 26b718d0f47d68a032c19eb351c59ff123400e96..1695388d3d98839fa09a2f0bc534f68a707c2a29 100644
--- a/pkg/barback/lib/src/transform.dart
+++ b/pkg/barback/lib/src/transform.dart
@@ -7,7 +7,6 @@ library barback.transform;
 import 'asset.dart';
 import 'asset_set.dart';
 import 'base_transform.dart';
-import 'transform_logger.dart';
 import 'transform_node.dart';
 
 /// While a [Transformer] represents a *kind* of transformation, this defines
@@ -18,10 +17,10 @@ import 'transform_node.dart';
 /// the transformation. It lets the [Transformer] access inputs and generate
 /// outputs.
 class Transform extends BaseTransform {
-  final AssetSet _outputs;
+  final _outputs = new AssetSet();
 
-  Transform(TransformNode node, this._outputs, LogFunction logFunction)
-    : super(node, logFunction);
+  Transform._(TransformNode node)
+    : super(node);
 
   /// Stores [output] as the output created by this transformation.
   ///
@@ -32,3 +31,14 @@ class Transform extends BaseTransform {
     _outputs.add(output);
   }
 }
+
+/// The controller for [Transform].
+class TransformController extends BaseTransformController {
+  Transform get transform => super.transform;
+
+  /// The set of assets that the transformer has emitted.
+  AssetSet get outputs => transform._outputs;
+
+  TransformController(TransformNode node)
+      : super(new Transform._(node));
+}