First pass at build dependency graph for barback.

pkg/barback/lib/src/asset_node.dart

Index: pkg/barback/lib/src/asset_node.dart
diff --git a/pkg/barback/lib/src/asset_node.dart b/pkg/barback/lib/src/asset_node.dart
new file mode 100644
index 0000000000000000000000000000000000000000..6fd86c622d7fab8058c23f134104e2f02690c719
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+++ b/pkg/barback/lib/src/asset_node.dart
@@ -0,0 +1,34 @@
+// 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;
+
+import 'dart:async';
+
+import 'asset.dart';
+import 'asset_graph.dart';
+import 'asset_id.dart';
+import 'phase.dart';
+import 'transform_node.dart';
+
+/// Describes an asset and its relationship to the build dependency graph.
+///
+/// Keeps a cache of the last asset that was built for this node (i.e. for this
+/// node's ID and phase) and tracks which transforms depend on it.
+class AssetNode {
+  final AssetId id;
+  Asset asset;
+
+  /// The [TransformNode]s that consume this node's asset as an input.
+  final consumers = new Set<TransformNode>();
+
+  AssetNode(this.id);
+
+  /// Updates this node's generated asset value and marks all transforms that
+  /// use this as dirty.
+  void updateAsset(Asset asset) {
+    this.asset = asset;
+    consumers.forEach((consumer) => consumer.dirty());
+  }
+}