Barback transforms now pass through the primary input by default.

pkg/barback/lib/src/asset_node_set.dart

Index: pkg/barback/lib/src/asset_node_set.dart
diff --git a/pkg/barback/lib/src/asset_node_set.dart b/pkg/barback/lib/src/asset_node_set.dart
new file mode 100644
index 0000000000000000000000000000000000000000..be897b951214f2d9cf16ee0383bcc057ecde6c73
--- /dev/null
+++ b/pkg/barback/lib/src/asset_node_set.dart
@@ -0,0 +1,24 @@
+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library barback.asset_node_set;
+
+import 'package:collection/collection.dart';
+
+import 'asset_node.dart';
+
+/// A set of [AssetNode]s that automatically ensures that nodes are removed from
+/// the set as soon as they're marked as [AssetState.REMOVED].
+class AssetNodeSet extends DelegatingSet<AssetNode> {
+  AssetNodeSet()
+      : super(new Set());
+
+  bool add(AssetNode node) {
+    if (node.state.isRemoved) return false;
+    node.whenRemoved(() => super.remove(node));
+    return super.add(node);
+  }
+
+  void addAll(Iterable<AssetNode> nodes) => nodes.forEach(add);
+}