diff -u b/core/modules/system/lib/Drupal/system/Controller/BatchController.php b/core/modules/system/lib/Drupal/system/Controller/BatchController.php
--- b/core/modules/system/lib/Drupal/system/Controller/BatchController.php
+++ b/core/modules/system/lib/Drupal/system/Controller/BatchController.php
@@ -21,7 +21,7 @@
    * Returns a system batch page.
    *
    * @return mixed
-   *   A Response object or page element.
+   *   A Symfony\Component\HttpFoundation\Response object or page element.
    *
    */
   public function batchPage(Request $request) {
diff -u b/core/update.php b/core/update.php
--- b/core/update.php
+++ b/core/update.php
@@ -396,6 +396,8 @@
   }
 }
 
+// @todo Refactor this.
+
 // Some unavoidable errors happen because the database is not yet up-to-date.
 // Our custom error handler is not yet installed, so we just suppress them.
 ini_set('display_errors', FALSE);