Factor out an input-handling class from Phase in barback.

pkg/barback/lib/src/transform_node.dart

Index: pkg/barback/lib/src/transform_node.dart
diff --git a/pkg/barback/lib/src/transform_node.dart b/pkg/barback/lib/src/transform_node.dart
index 39914a183f871fd3f76d9d6fe9df3ac019885a83..578b2fa20c8301b95cce949507f8ff91e626a5bd 100644
--- a/pkg/barback/lib/src/transform_node.dart
+++ b/pkg/barback/lib/src/transform_node.dart
@@ -132,7 +132,7 @@ class TransformNode {
       // Don't allow partial results from a failed transform.
       newOutputs.clear();
     }).then((_) {
-      if (_isDirty) return [];
+      if (_isDirty) return new Set();
 
       return _adjustOutputs(newOutputs);
     });