diff -u b/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
--- b/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -5,7 +5,6 @@
  * Functions that need to be loaded on every Drupal request.
  */
 
-use Drupal\Component\Render\FormattableMarkup;
 use Drupal\Component\Utility\Crypt;
 use Drupal\Component\Utility\Html;
 use Drupal\Component\Utility\SafeMarkup;
@@ -228,34 +227,33 @@
       return $extension_list->getPathname($name);
     }
     catch (\InvalidArgumentException $e) {
-      // Catch the exception and trigger error to maintain existing behavior.
-      trigger_error(new FormattableMarkup('The following @type is missing from the file system: @name', ['@type' => $type, '@name' => $name]), E_USER_WARNING);
+      // Catch the exception. This will result in triggering an error.
     }
   }
+  else {
 
-  if (!isset($files[$type])) {
-    $files[$type] = [];
-  }
+    if (!isset($files[$type])) {
+      $files[$type] = [];
+    }
 
-  if (isset($filename)) {
-    $files[$type][$name] = $filename;
-  }
-  elseif (!isset($files[$type][$name])) {
-    // If still unknown, retrieve the file list prepared in state by
-    // system_rebuild_module_data() and
-    // \Drupal\Core\Extension\ThemeHandlerInterface::rebuildThemeData().
-    if (!isset($files[$type][$name]) && \Drupal::hasService('state')) {
-      $files[$type] += \Drupal::state()->get('system.' . $type . '.files', []);
+    if (isset($filename)) {
+      $files[$type][$name] = $filename;
     }
-    // If still unknown, create a user-level error message.
-    if (!isset($files[$type][$name])) {
-      trigger_error(SafeMarkup::format('The following @type is missing from the file system: @name', ['@type' => $type, '@name' => $name]), E_USER_WARNING);
+    elseif (!isset($files[$type][$name])) {
+      // If still unknown, retrieve the file list prepared in state by
+      // system_rebuild_module_data() and
+      // \Drupal\Core\Extension\ThemeHandlerInterface::rebuildThemeData().
+      if (!isset($files[$type][$name]) && \Drupal::hasService('state')) {
+        $files[$type] += \Drupal::state()->get('system.' . $type . '.files', []);
+      }
     }
-  }
 
-  if (isset($files[$type][$name])) {
-    return $files[$type][$name];
+    if (isset($files[$type][$name])) {
+      return $files[$type][$name];
+    }
   }
+  // If the filename is still unknown, create a user-level error message.
+  trigger_error(SafeMarkup::format('The following @type is missing from the file system: @name', ['@type' => $type, '@name' => $name]), E_USER_WARNING);
 }
 
 /**
diff -u b/core/includes/install.core.inc b/core/includes/install.core.inc
--- b/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -1071,7 +1071,7 @@
   file_ensure_htaccess();
 
   // Prime the drupal_get_filename() static cache with the user module's
-  // location.
+  // exact location.
   // @todo Remove as part of https://www.drupal.org/node/2186491
   drupal_get_filename('module', 'user', 'core/modules/user/user.info.yml');
 
diff -u b/core/includes/install.inc b/core/includes/install.inc
--- b/core/includes/install.inc
+++ b/core/includes/install.inc
@@ -626,7 +626,7 @@
 
   // Before having installed the system module and being able to do a module
   // rebuild, prime the ModuleExtensionList static cache with the module's
-  // exact location.
+  // location.
   // @todo Try to install system as any other module, see
   //   https://www.drupal.org/node/2719315.
   \Drupal::service('extension.list.module')->setPathname('system', 'core/modules/system/system.info.yml');
reverted:
--- b/core/lib/Drupal/Core/DrupalKernel.php
+++ a/core/lib/Drupal/Core/DrupalKernel.php
@@ -841,10 +841,7 @@
     if (isset($this->container)) {
       // Save the id of the currently logged in user.
       if ($this->container->initialized('current_user')) {
+        $current_user_id = $this->container->get('current_user')->id();
-        /** @var \Drupal\Core\Session\AccountProxyInterface $current_user */
-        $current_user = $this->container->get('current_user');
-        // Ensure to not accidentally initialize the user.
-        $current_user_id = $current_user->hasAccount() ? $current_user->id() : 0;
       }
 
       // If there is a session, close and save it.
diff -u b/core/lib/Drupal/Core/Extension/ExtensionList.php b/core/lib/Drupal/Core/Extension/ExtensionList.php
--- b/core/lib/Drupal/Core/Extension/ExtensionList.php
+++ b/core/lib/Drupal/Core/Extension/ExtensionList.php
@@ -522,7 +522,7 @@
 
   /**
    * Gets the path to an extension of a specific type (module, theme, etc.).
-   * 
+   *
    * The path is the directory in which the .info file is located. This name is
    * coming from \SplFileInfo.
    *
reverted:
--- b/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
+++ a/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
@@ -306,9 +306,6 @@
    * @return string
    *   Returns the human readable name of the module or the machine name passed
    *   in if no matching module is found.
-   *
-   * @deprecated in Drupal 8.4.0, will be removed before Drupal 9.0.0.
-   *   Use \Drupal::service('extension.list.module')->getName() instead.
    */
   public function getName($module);
 
reverted:
--- b/core/lib/Drupal/Core/Session/AccountProxy.php
+++ a/core/lib/Drupal/Core/Session/AccountProxy.php
@@ -52,13 +52,6 @@
     date_default_timezone_set(drupal_get_user_timezone());
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  public function hasAccount() {
-    return isset($this->account);
-  }
-
   /**
    * {@inheritdoc}
    */
reverted:
--- b/core/lib/Drupal/Core/Session/AccountProxyInterface.php
+++ a/core/lib/Drupal/Core/Session/AccountProxyInterface.php
@@ -6,7 +6,6 @@
  * Defines an interface for a service which has the current account stored.
  *
  * @ingroup user_api
- * @internal
  */
 interface AccountProxyInterface extends AccountInterface {
 
@@ -44,12 +43,4 @@
    */
   public function setInitialAccountId($account_id);
 
-  /**
-   * Checks whether an account is currently wrapped.
-   *
-   * @return bool
-   *   TRUE, if an account is currently wrapped. FALSE otherwise.
-   */
-  public function hasAccount();
-
 }
diff -u b/core/modules/system/system.module b/core/modules/system/system.module
--- b/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -995,8 +995,11 @@
  *
  * @deprecated in Drupal 8.4.x and will be removed before Drupal 9.0.0. This
  *   function is no longer used in Drupal core.
+ *
+ * @see https://www.drupal.org/node/2709919
  */
 function _system_rebuild_module_data_ensure_required($module, &$modules) {
+  @trigger_error("_system_rebuild_module_data_ensure_required() is deprecated in Drupal 8.5.0 and will be removed before Drupal 9.0.0. This function is no longer used in Drupal core. See https://www.drupal.org/node/2709919", E_USER_DEPRECATED);
   if (!empty($module->info['required'])) {
     foreach ($module->info['dependencies'] as $dependency) {
       $dependency_name = ModuleHandler::parseDependency($dependency)['name'];
@@ -1016,12 +1019,14 @@
  * @return \Drupal\Core\Extension\Extension[]
  *   An associative array of module information.
  *
- * @deprecated in Drupal 8.4.x and will be removed before Drupal 9.0.0.
+ * @deprecated in Drupal 8.5.0 and will be removed before Drupal 9.0.0.
  *   Use \Drupal::service('extension.list.module')->reset()->list()
  *   instead. Note: You probably don't need the reset() method.
+ *
+ * @see https://www.drupal.org/node/2709919
  */
 function _system_rebuild_module_data() {
-  @trigger_error("_system_rebuild_module_data() is deprecated in Drupal 8.4.0 and will be removed before Drupal 9.0.0. Instead, you should use \\Drupal::service('extension.list.module')->reset()->list(). See https://www.drupal.org/node/2709919", E_USER_DEPRECATED);
+  @trigger_error("_system_rebuild_module_data() is deprecated in Drupal 8.5.0 and will be removed before Drupal 9.0.0. Instead, you should use \\Drupal::service('extension.list.module')->reset()->list(). See https://www.drupal.org/node/2709919", E_USER_DEPRECATED);
   return \Drupal::service('extension.list.module')->reset()->list();
 }
 
@@ -1030,11 +1035,8 @@
  *
  * @return \Drupal\Core\Extension\Extension[]
  *   Array of all available modules and their data.
- *
- * @deprecated in Drupal 8.4.x and will be removed before Drupal 9.0.0.
  */
 function system_rebuild_module_data() {
-  @trigger_error("system_rebuild_module_data() is deprecated in Drupal 8.4.0 and will be removed before Drupal 9.0.0. Instead, you should use \\Drupal::service('extension.list.module')->reset()->list(). See https://www.drupal.org/node/2709919", E_USER_DEPRECATED);
   return \Drupal::service('extension.list.module')->reset()->list();
 }
 
reverted:
--- b/core/tests/Drupal/Tests/Core/Session/AccountProxyTest.php
+++ a/core/tests/Drupal/Tests/Core/Session/AccountProxyTest.php
@@ -42,17 +42,6 @@
     $account_proxy->setInitialAccountId(1);
   }
 
-  /**
-   * @covers ::hasAccount
-   */
-  public function testHasAccount() {
-    $account_proxy = new AccountProxy();
-    $this->assertFalse($account_proxy->hasAccount());
-
-    $account_proxy->getAccount();
-    $this->assertTrue($account_proxy->hasAccount());
-  }
-
 }
 
 namespace Drupal\Core\Session;
only in patch2:
unchanged:
--- a/core/modules/filter/tests/src/Functional/FilterFormTest.php
+++ b/core/modules/filter/tests/src/Functional/FilterFormTest.php
@@ -72,7 +72,7 @@ public function testFilterForm() {
     // @see https://www.drupal.org/node/2387983
     \Drupal::service('module_installer')->install(['filter_test_plugin']);
     // Force rebuild module data.
-    _system_rebuild_module_data();
+    \Drupal::service('extension.list.module')->reset();
   }
 
   /**
only in patch2:
unchanged:
--- a/core/modules/update/update.module
+++ b/core/modules/update/update.module
@@ -625,7 +625,6 @@ function template_preprocess_update_last_check(&$variables) {
  * version of Drupal core.
  *
  * @see \Drupal\Core\Extension\ExtensionDiscovery
- * @see _system_rebuild_module_data()
  */
 function update_verify_update_archive($project, $archive_file, $directory) {
   $errors = [];
only in patch2:
unchanged:
--- a/core/tests/Drupal/KernelTests/Core/Bootstrap/GetFilenameTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Bootstrap/GetFilenameTest.php
@@ -49,20 +49,24 @@ public function testDrupalGetFilename() {
     // a fixed location and naming.
     $this->assertIdentical(drupal_get_filename('profile', 'testing'), 'core/profiles/testing/testing.info.yml');
 
-    // Generate a non-existing module name.
-    $non_existing_module = uniqid("", TRUE);
-
     // Set a custom error handler so we can ignore the file not found error.
     set_error_handler(function ($severity, $message, $file, $line) {
       // Skip error handling if this is a "file not found" error.
       if (strstr($message, 'is missing from the file system:')) {
-        \Drupal::state()->set('get_filename_test_triggered_error', TRUE);
+        \Drupal::state()->set('get_filename_test_triggered_error', $message);
         return;
       }
       throw new \ErrorException($message, 0, $severity, $file, $line);
     });
-    $this->assertNull(drupal_get_filename('module', $non_existing_module), 'Searching for an item that does not exist returns NULL.');
-    $this->assertTrue(\Drupal::state()->get('get_filename_test_triggered_error'), 'Searching for an item that does not exist triggers an error.');
+    $this->assertNull(drupal_get_filename('module', 'there_is_a_module_for_that'), 'Searching for an item that does not exist returns NULL.');
+    $this->assertEquals('The following module is missing from the file system: there_is_a_module_for_that', \Drupal::state()->get('get_filename_test_triggered_error'));
+
+    $this->assertNull(drupal_get_filename('theme', 'there_is_a_theme_for_you'), 'Searching for an item that does not exist returns NULL.');
+    $this->assertEquals('The following theme is missing from the file system: there_is_a_theme_for_you', \Drupal::state()->get('get_filename_test_triggered_error'));
+
+    $this->assertNull(drupal_get_filename('profile', 'there_is_an_install_profile_for_you'), 'Searching for an item that does not exist returns NULL.');
+    $this->assertEquals('The following profile is missing from the file system: there_is_an_install_profile_for_you', \Drupal::state()->get('get_filename_test_triggered_error'));
+
     // Restore the original error handler.
     restore_error_handler();
   }