Support cross-package transforms in barback.

pkg/barback/lib/src/barback.dart

Index: pkg/barback/lib/src/barback.dart
diff --git a/pkg/barback/lib/src/barback.dart b/pkg/barback/lib/src/barback.dart
index 2797c1a75fd93bad71c1752cffa750b1bf155d28..9de1b70e87c19bab470902242e4e370fb34c6b04 100644
--- a/pkg/barback/lib/src/barback.dart
+++ b/pkg/barback/lib/src/barback.dart
@@ -8,7 +8,7 @@ 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';
 
@@ -69,7 +69,8 @@ class Barback {
   /// 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) =>
+    _graph.getAssetNode(id).then((node) => node.whenAvailable);
 
   /// Adds [sources] to the graph's known set of source assets.
   ///