Transition barback's infrastructure to an aggregate-based model.

pkg/barback/lib/src/asset/asset_node_set.dart

Index: pkg/barback/lib/src/asset/asset_node_set.dart
diff --git a/pkg/barback/lib/src/asset/asset_node_set.dart b/pkg/barback/lib/src/asset/asset_node_set.dart
index 4474866553473ea96eb5754b27e51044a5527178..a1fc17bebcc9952dd07c2e10173244f2b4dd4afb 100644
--- a/pkg/barback/lib/src/asset/asset_node_set.dart
+++ b/pkg/barback/lib/src/asset/asset_node_set.dart
@@ -6,19 +6,37 @@ library barback.asset.asset_node_set;
 
 import 'package:collection/collection.dart';
 
+import 'asset_id.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].
+///
+/// Asset nodes may be accessed by their ids. This means that only one node with
+/// a given id may be stored in the set at a time.
 class AssetNodeSet extends DelegatingSet<AssetNode> {
+  // TODO(nweiz): Use DelegatingMapSet when issue 18705 is fixed.
+  /// A map from asset ids to assets in the set.
+  final _assetsById = new Map<AssetId, AssetNode>();
+
   AssetNodeSet()
       : super(new Set());
 
+  /// Returns the asset node in the set with [id], or `null` if none exists.
+  AssetNode operator [](AssetId id) => _assetsById[id];
+
   bool add(AssetNode node) {
     if (node.state.isRemoved) return false;
-    node.whenRemoved(() => super.remove(node));
+    node.whenRemoved(() {
+      super.remove(node);
+      _assetsById.remove(node.id);
+    });
+    _assetsById[node.id] = node;
     return super.add(node);
   }
 
+  /// Returns whether an asset node with the given [id] is in the set.
+  bool containsId(AssetId id) => _assetsById.containsKey(id);
+
   void addAll(Iterable<AssetNode> nodes) => nodes.forEach(add);
 }