Issue #2890690: MigrateLookup plugin has inconsistent return values.

core/modules/aggregator/migrations/d6_aggregator_item.yml

@@ -9,9 +9,13 @@ source:
 process:
   iid: iid
   fid:
-    plugin: migration_lookup
-    migration: d6_aggregator_feed
-    source: fid
+    - plugin: migration_lookup
+      migration: d6_aggregator_feed
+      source: fid
+      return_associative: TRUE
+    - plugin: extract
+      index:
+        - fid
   title: title
   link: link
   author: author