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

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 27c4d472f2b677e6c3083532ff58c511234006fa..3dcb23e66d166996a44b27bd55c3606041e794aa 100644
--- a/pkg/barback/lib/src/barback.dart
+++ b/pkg/barback/lib/src/barback.dart
@@ -73,7 +73,7 @@ class Barback {
   Future<Asset> getAssetById(AssetId id) {
     return _graph.getAssetNode(id).then((node) {
       if (node == null) throw new AssetNotFoundException(id);
-      return node.whenAvailable;
+      return node.asset;
     });
   }