reverted:
--- b/core/modules/system/system.install
+++ a/core/modules/system/system.install
@@ -2142,17 +2142,6 @@
 }
 
 /**
- * Moves drupal_private_key variable to state.
- *
- * @ingroup config_upgrade
- */
-function system_update_8050() {
-  update_variables_to_state(array(
-    'drupal_private_key' => 'system.private_key',
-  ));
-}
-
-/**
  * @} End of "defgroup updates-7.x-to-8.x".
  * The next series of updates should start at 9000.
  */
only in patch2:
unchanged:
--- a/core/includes/update.inc
+++ b/core/includes/update.inc
@@ -400,6 +400,12 @@ function update_prepare_d8_bootstrap() {
       $theme_config->save();
       $disabled_themes->save();
 
+      // Migrate the private key to state. This is used to create the token for
+      // the upgrade batch so needs to be be done before the upgrade has begun.
+      update_variables_to_state(array(
+        'drupal_private_key' => 'system.private_key',
+      ));
+
       // Update the dynamic include paths that might be used before running the
       // proper update functions.
       update_prepare_stored_includes();