Only pass an AssetId to isPrimary and declareOutputs.

sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart

Index: sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
index 89859d6fbfc0d963605c13610bc9df0e829c3cf0..c5cb4b9ca815c643827f293f300d21647f676644 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
@@ -91,10 +91,10 @@ class _ForeignTransformer extends Transformer {
       : _port = map['port'],
         _toString = map['toString'];
 
-  Future<bool> isPrimary(Asset asset) {
+  Future<bool> isPrimary(AssetId id) {
     return call(_port, {
       'type': 'isPrimary',
-      'asset': serializeAsset(asset)
+      'id': serializeId(id)
     });
   }