reverted:
--- b/core/lib/Drupal/Core/Session/AccountEvents.php
+++ /dev/null
@@ -1,28 +0,0 @@
-<?php
-
-namespace Drupal\Core\Session;
-
-/**
- * Defines events for the account system.
- *
- * @see \Drupal\Core\Session\AccountSetEvent
- */
-final class AccountEvents {
-
-  /**
-   * Name of the event fired when the current user is set.
-   *
-   * This event allows modules to perform an action whenever the current user is
-   * set. The event listener receives an \Drupal\Core\Session\AccountSetEvent
-   * instance.
-   *
-   * @Event
-   *
-   * @see \Drupal\Core\Session\AccountSetEvent
-   * @see \Drupal\Core\Session\AccountProxyInterface::setAccount()
-   *
-   * @var string
-   */
-  const SET_USER = 'account.set';
-
-}
diff -u b/core/lib/Drupal/Core/Session/AccountProxy.php b/core/lib/Drupal/Core/Session/AccountProxy.php
--- b/core/lib/Drupal/Core/Session/AccountProxy.php
+++ b/core/lib/Drupal/Core/Session/AccountProxy.php
@@ -71,7 +71,7 @@
     }
     $this->account = $account;
     $this->id = $account->id();
-    $this->eventDispatcher->dispatch(AccountEvents::SET_USER, new AccountSetEvent($account));
+    $this->eventDispatcher->dispatch(AccountSetEvent::SET_USER, new AccountSetEvent($account));
   }
 
   /**
diff -u b/core/lib/Drupal/Core/Session/AccountSetEvent.php b/core/lib/Drupal/Core/Session/AccountSetEvent.php
--- b/core/lib/Drupal/Core/Session/AccountSetEvent.php
+++ b/core/lib/Drupal/Core/Session/AccountSetEvent.php
@@ -7,7 +7,21 @@
 /**
  * Event fired when an account is set for the current session.
  */
-class AccountSetEvent extends Event {
+final class AccountSetEvent extends Event {
+
+  /**
+   * Name of the event fired when the current user is set.
+   *
+   * This event allows modules to perform an action whenever the current user is
+   * set. An event listener receives an the instance of that class.
+   *
+   * @Event
+   *
+   * @see \Drupal\Core\Session\AccountProxyInterface::setAccount()
+   *
+   * @var string
+   */
+  const SET_USER = 'account.set';
 
   /**
    * The set account.
diff -u b/core/modules/system/src/TimeZoneResolver.php b/core/modules/system/src/TimeZoneResolver.php
--- b/core/modules/system/src/TimeZoneResolver.php
+++ b/core/modules/system/src/TimeZoneResolver.php
@@ -5,8 +5,8 @@
 use Drupal\Core\Config\ConfigCrudEvent;
 use Drupal\Core\Config\ConfigEvents;
 use Drupal\Core\Config\ConfigFactoryInterface;
-use Drupal\Core\Session\AccountEvents;
 use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\Session\AccountSetEvent;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 use Symfony\Component\HttpKernel\KernelEvents;
 
@@ -76,7 +76,7 @@
     // The priority for this must run directly after the authentication
     // subscriber.
     $events[KernelEvents::REQUEST][] = ['setDefaultTimeZone', 299];
-    $events[AccountEvents::SET_USER][] = ['setDefaultTimeZone'];
+    $events[AccountSetEvent::SET_USER][] = ['setDefaultTimeZone'];
     return $events;
   }