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..06b2952a90040cba2f058d2f208617019a1f01f9
--- /dev/null
+++ b/pkg/barback/lib/src/asset_node.dart
@@ -0,0 +1,32 @@
+// 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 '../barback.dart';
+import 'asset_graph.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 built asset and tracks which transforms depend

[nweiz, 2013/06/18 23:14:46]

I don't understand what "last built asset" means. Is it the last asset that was
ever built? The last asset with the given id that was built? Something else?

[Bob Nystrom, 2013/06/20 00:23:59]

Clarified.

+/// on it.
+class AssetNode {
+  final AssetId id;
+  Asset asset;
+
+  /// The [_TransformNode]s that consume this node's asset as an input.

[nweiz, 2013/06/18 23:14:46]

Remove "_"

[Bob Nystrom, 2013/06/20 00:23:59]

Done.

+  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.isDirty = true);
+  }
+}