Reorganize barback's source files.

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.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();
-  }
-}