diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index 34e68241ab..aa1425213c 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -1557,6 +1557,8 @@ function install_bootstrap_full() {
   $session = \Drupal::service('session');
   \Drupal::request()->setSession($session);
   $session->start();
+  // Ensure the session is maintained throughout the install.
+  $_SESSION['_drupap_installing'] = TRUE;
 }
 
 /**
diff --git a/core/lib/Drupal/Core/Session/MetadataBag.php b/core/lib/Drupal/Core/Session/MetadataBag.php
index e7a65a58b0..dad5dabdf1 100644
--- a/core/lib/Drupal/Core/Session/MetadataBag.php
+++ b/core/lib/Drupal/Core/Session/MetadataBag.php
@@ -51,8 +51,7 @@ public function getCsrfTokenSeed() {
   /**
    * Clear the CSRF token seed.
    */
-  public function stampNew($lifetime = null)
-  {
+  public function stampNew($lifetime = NULL) {
     parent::stampNew($lifetime);
     unset($this->meta[static::CSRF_TOKEN_SEED]);
   }
diff --git a/core/lib/Drupal/Core/Session/SessionManager.php b/core/lib/Drupal/Core/Session/SessionManager.php
index a51d301bdf..0fcb915f25 100644
--- a/core/lib/Drupal/Core/Session/SessionManager.php
+++ b/core/lib/Drupal/Core/Session/SessionManager.php
@@ -2,7 +2,6 @@
 
 namespace Drupal\Core\Session;
 
-use Drupal\Component\Utility\Crypt;
 use Drupal\Core\Database\Connection;
 use Drupal\Core\DependencyInjection\DependencySerializationTrait;
 use Symfony\Component\HttpFoundation\RequestStack;
diff --git a/core/lib/Drupal/Core/TempStore/SharedTempStoreFactory.php b/core/lib/Drupal/Core/TempStore/SharedTempStoreFactory.php
index 4267edef24..a7b5b7d72b 100644
--- a/core/lib/Drupal/Core/TempStore/SharedTempStoreFactory.php
+++ b/core/lib/Drupal/Core/TempStore/SharedTempStoreFactory.php
@@ -2,7 +2,6 @@
 
 namespace Drupal\Core\TempStore;
 
-use Drupal\Component\Utility\Crypt;
 use Drupal\Core\KeyValueStore\KeyValueExpirableFactoryInterface;
 use Drupal\Core\Lock\LockBackendInterface;
 use Symfony\Component\HttpFoundation\RequestStack;