Support cross-package transforms in barback.

pkg/barback/lib/src/package_graph.dart

Index: pkg/barback/lib/src/package_graph.dart
diff --git a/pkg/barback/lib/src/package_graph.dart b/pkg/barback/lib/src/package_graph.dart
index 4dc2719221467bd29fe50a07041910903eb8e35a..9f876eb95c59d62ac2971e87f1014e15e65fdf53 100644
--- a/pkg/barback/lib/src/package_graph.dart
+++ b/pkg/barback/lib/src/package_graph.dart
@@ -11,6 +11,8 @@ import 'package:stack_trace/stack_trace.dart';
 import 'asset.dart';
 import 'asset_cascade.dart';
 import 'asset_id.dart';
+import 'asset_node.dart';
+import 'build_result.dart';
 import 'errors.dart';
 import 'package_provider.dart';
 import 'utils.dart';
@@ -80,14 +82,14 @@ class PackageGraph {
     _errors = mergeStreams(_cascades.values.map((cascade) => cascade.errors));
   }
 
-  /// Gets the asset identified by [id].
+  /// Gets the asset node 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) {
+  Future<AssetNode> getAssetNode(AssetId id) {
     var cascade = _cascades[id.package];
-    if (cascade != null) return cascade.getAssetById(id);
+    if (cascade != null) return cascade.getAssetNode(id);
     return new Future.error(
         new AssetNotFoundException(id),
         new Trace.current().vmTrace);