Support cross-package transforms in barback.

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';
-import 'asset_cascade.dart';
+import 'build_result.dart';
 import 'package_graph.dart';
 import 'package_provider.dart';
 
 /// A general-purpose asynchronous build dependency graph manager.
 ///
 /// It consumes source assets (including Dart files) in a set of packages,
 /// runs transformations on them, and then tracks which sources have been
 /// modified and which transformations need to be re-run.
 ///
 /// To do this, you give barback a [PackageProvider] which can yield a set of
@@ skipping 40 matching lines @@
   Stream get errors => _graph.errors;
 
   Barback(PackageProvider provider)
       : _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) => _graph.getAssetById(id);
+  Future<Asset> getAssetById(AssetId id) {
+    return _graph.getAssetNode(id).then((node) {
+      if (node == null) throw new AssetNotFoundException(id);
+      return node.whenAvailable;
+    });
+  }
 
   /// 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);
 }