Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/barback/src/asset/asset_node_set.dart

+// 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.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);
+      _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);
+}