Support passing barback assets through phases in which they're unused.

pkg/barback/lib/src/barback.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.barback;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 55 matching lines @@
       : _graph = new PackageGraph(provider);
 
   /// Gets the asset identified by [id].
   ///
   /// If [id] is for a generated or transformed asset, this will wait until
   /// it has been created and return it. If the asset cannot be found, throws
   /// [AssetNotFoundException].
   Future<Asset> getAssetById(AssetId id) {
     return _graph.getAssetNode(id).then((node) {
       if (node == null) throw new AssetNotFoundException(id);
-      return node.whenAvailable;
+      return node.asset;
     });
   }
 
   /// Adds [sources] to the graph's known set of source assets.
   ///
   /// Begins applying any transforms that can consume any of the sources. If a
   /// given source is already known, it is considered modified and all
   /// transforms that use it will be re-applied.
   void updateSources(Iterable<AssetId> sources) =>
       _graph.updateSources(sources);
 
   /// Removes [removed] from the graph's known set of source assets.
   void removeSources(Iterable<AssetId> removed) =>
       _graph.removeSources(removed);
 }