First pass at build dependency graph for barback.

pkg/barback/lib/src/change_batch.dart

Index: pkg/barback/lib/src/change_batch.dart
diff --git a/pkg/barback/lib/src/change_batch.dart b/pkg/barback/lib/src/change_batch.dart
new file mode 100644
index 0000000000000000000000000000000000000000..10759c2da15dd8c76c039663643d2769088ba2d7
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+++ b/pkg/barback/lib/src/change_batch.dart
@@ -0,0 +1,33 @@
+// 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.change_batch;
+
+import '../barback.dart';
+
+/// Represents a batch of source asset changes: additions, removals and
+/// modifications.
+class ChangeBatch {
+  /// The assets that have been added or modified in this batch.
+  final updated = new Set<AssetId>();
+
+  /// The assets that have been removed in this batch.
+  final removed = new Set<AssetId>();
+
+  /// Adds the updated [assets] to this batch.
+  void update(Iterable<AssetId> assets) {
+    updated.addAll(assets);
+
+    // If they were previously removed, they are back now.
+    removed.removeAll(assets);
+  }
+
+  /// Removes [assets] from this batch.
+  void remove(Iterable<AssetId> assets) {
+    removed.addAll(assets);
+
+    // If they were previously updated, they are gone now.
+    updated.removeAll(assets);
+  }
+}