Factor a PhaseOutput class out of Phase.

pkg/barback/lib/src/asset_forwarder.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_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(node.id, node.transform) {
+      : _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();
       }
     });
-
-    if (node.state.isAvailable) {
-      _controller.setAvailable(node.asset);
-    } else if (node.state.isRemoved) {
-      close();
-    }
   }
 
   /// Closes the forwarder and marks [node] as removed.
   void close() {
     if (_controller.node.state.isRemoved) return;
     _subscription.cancel();
     _controller.setRemoved();
   }
 }