Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/barback/src/asset/asset_forwarder.dart

Index: observatory_pub_packages/barback/src/asset/asset_forwarder.dart
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--- observatory_pub_packages/barback/src/asset/asset_forwarder.dart	(revision 0)
+++ observatory_pub_packages/barback/src/asset/asset_forwarder.dart	(working copy)
@@ -0,0 +1,49 @@
+// 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_forwarder;
+
+import 'dart:async';
+
+import 'asset_node.dart';
+
+/// A wrapper for an [AssetNode] that forwards events to a new node.
+///
+/// A forwarder is used when a class wants to forward an [AssetNode] that it
+/// gets as an input, but also wants to have control over when that node is
+/// marked as removed. The forwarder can be closed, thus removing its output
+/// node, without the original node having been removed.
+class AssetForwarder {
+  /// The subscription on the input node.
+  StreamSubscription _subscription;
+
+  /// The controller for the output node.
+  final AssetNodeController _controller;
+
+  /// The node to which events are forwarded.
+  AssetNode get node => _controller.node;
+
+  AssetForwarder(AssetNode node)
+      : _controller = new AssetNodeController.from(node) {
+    if (node.state.isRemoved) return;
+
+    _subscription = node.onStateChange.listen((state) {
+      if (state.isAvailable) {
+        _controller.setAvailable(node.asset);
+      } else if (state.isDirty) {
+        _controller.setDirty();
+      } else {
+        assert(state.isRemoved);
+        close();
+      }
+    });
+  }
+
+  /// Closes the forwarder and marks [node] as removed.
+  void close() {
+    if (_controller.node.state.isRemoved) return;
+    _subscription.cancel();
+    _controller.setRemoved();
+  }
+}