diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index 41cc436..ff47be3 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -1063,7 +1063,7 @@ function install_base_system(&$install_state) {
   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 --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 44e653f..3f53d04 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -840,10 +840,7 @@ protected function initializeContainer() {
     if (isset($this->container)) {
       // Save the id of the currently logged in user.
       if ($this->container->initialized('current_user')) {
-        /** @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;
+        $current_user_id = $this->container->get('current_user')->id();
       }
 
       // If there is a session, close and save it.
diff --git a/core/lib/Drupal/Core/Extension/ExtensionList.php b/core/lib/Drupal/Core/Extension/ExtensionList.php
index 7a54d3c..bb48eca 100644
--- a/core/lib/Drupal/Core/Extension/ExtensionList.php
+++ b/core/lib/Drupal/Core/Extension/ExtensionList.php
@@ -8,7 +8,7 @@
 /**
  * Provides available extensions.
  *
- * The extension list is per extension type, like module theme and profile.
+ * The extension list is per extension type (i.e. module, theme, or profile).
  */
 abstract class ExtensionList {
 
@@ -333,7 +333,7 @@ protected function doListExtensions() {
    * extension.
    *
    * @param string $extension_name
-   *   The name of an extension whose information shall be returned.
+   *   The machine name of an extension whose information shall be returned.
    *
    * @return mixed[]
    *   An associative array of extension information.
@@ -458,7 +458,7 @@ protected function recalculatePathnames() {
    * It is not recommended to call this method except in those rare cases.
    *
    * @param string $extension_name
-   *   The name of the extension for which the filename is requested.
+   *   The machine name of the extension for which the filename is requested.
    * @param string $pathname
    *   The pathname of the extension which is to be set explicitly rather
    *   than by consulting the dynamic extension listing.
@@ -472,8 +472,8 @@ public function setPathname($extension_name, $pathname) {
 
     // In the early installer the container is rebuilt multiple times. Therefore
     // we have to keep the added filenames across those rebuilds. This is not a
-    // final design, but rather just a workaround resolved at some point,
-    // hopefully.
+    // final design, but rather just a workaround that should be resolved at
+    // some point.
     if (!empty($GLOBALS['install_state'])) {
       static::$staticAddedPathNames[$extension_name] = $pathname;
     }
@@ -511,7 +511,7 @@ public function setPathname($extension_name, $pathname) {
    * depending on where the extension is located and what type it is.
    *
    * @param string $extension_name
-   *   The name of the extension for which the pathname is requested.
+   *   The machine name of the extension for which the pathname is requested.
    *
    * @return string
    *   The filename of the requested extension's .info.yml file.
@@ -540,7 +540,7 @@ public function getPathname($extension_name) {
    * Gets the path to an extension of a specific type (module, theme, etc.).
    *
    * @param string $extension_name
-   *   The name of the extension for which the path is requested.
+   *   The machine name of the extension for which the path is requested.
    *
    * @return string
    *   The Drupal-root-relative path to the specified extension.
diff --git a/core/lib/Drupal/Core/Extension/ModuleExtensionList.php b/core/lib/Drupal/Core/Extension/ModuleExtensionList.php
index 00f154b..cd535a0 100644
--- a/core/lib/Drupal/Core/Extension/ModuleExtensionList.php
+++ b/core/lib/Drupal/Core/Extension/ModuleExtensionList.php
@@ -5,15 +5,13 @@
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Config\ConfigFactoryInterface;
 use Drupal\Core\State\StateInterface;
-use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
 
 /**
  * Provides a list of available modules.
  */
 class ModuleExtensionList extends ExtensionList {
 
-  use StringTranslationTrait;
-
   /**
    * {@inheritdoc}
    */
@@ -219,7 +217,7 @@ protected function ensureRequiredDependencies(Extension $module, array $modules
         $dependency_name = ModuleHandler::parseDependency($dependency)['name'];
         if (!isset($modules[$dependency_name]->info['required'])) {
           $modules[$dependency_name]->info['required'] = TRUE;
-          $modules[$dependency_name]->info['explanation'] = $this->t('Dependency of required module @module', ['@module' => $module->info['name']]);
+          $modules[$dependency_name]->info['explanation'] = new TranslatableMarkup('Dependency of required module @module', ['@module' => $module->info['name']]);
           // Ensure any dependencies it has are required.
           $this->ensureRequiredDependencies($modules[$dependency_name], $modules);
         }
diff --git a/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php b/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
index 0253358..6b2738c 100644
--- a/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
+++ b/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
@@ -306,9 +306,6 @@ public function getModuleDirectories();
    * @return string
    *   Returns the human readable name of the module or the machine name passed
    *   in if no matching module is found.
-   *
-   * @deprecated in Drupal 8.4.0, will be removed before Drupal 9.0.0.
-   *   Use \Drupal::service('extension.list.module')->getName() instead.
    */
   public function getName($module);
 
diff --git a/core/lib/Drupal/Core/Extension/module.api.php b/core/lib/Drupal/Core/Extension/module.api.php
index 944d354..fde4e78 100644
--- a/core/lib/Drupal/Core/Extension/module.api.php
+++ b/core/lib/Drupal/Core/Extension/module.api.php
@@ -132,10 +132,10 @@ function hook_module_implements_alter(&$implementations, $hook) {
 /**
  * Alter the information parsed from module and theme .info.yml files.
  *
- * This hook is invoked in _system_rebuild_module_data() and in
- * \Drupal\Core\Extension\ThemeHandlerInterface::rebuildThemeData(). A module
- * may implement this hook in order to add to or alter the data generated by
- * reading the .info.yml file with \Drupal\Core\Extension\InfoParser.
+ * This hook is invoked in ExtensionList::doListExtensions() and in
+ * ThemeHandlerInterface::rebuildThemeData(). A module may implement this hook
+ * in order to add to or alter the data generated by reading the .info.yml file
+ * with \Drupal\Core\Extension\InfoParser.
  *
  * Using implementations of this hook to make modules required by setting the
  * $info['required'] key is discouraged. Doing so will slow down the module
@@ -150,7 +150,9 @@ function hook_module_implements_alter(&$implementations, $hook) {
  *   Either 'module' or 'theme', depending on the type of .info.yml file that
  *   was passed.
  *
+ * @see \Drupal\Core\Extension\ExtensionList::doListExtensions()
  * @see \Drupal\Core\Extension\ModuleUninstallValidatorInterface
+ * @see \Drupal\Core\Extension\ThemeHandlerInterface::rebuildThemeData()
  */
 function hook_system_info_alter(array &$info, \Drupal\Core\Extension\Extension $file, $type) {
   // Only fill this in if the .info.yml file does not define a 'datestamp'.
diff --git a/core/lib/Drupal/Core/Session/AccountProxy.php b/core/lib/Drupal/Core/Session/AccountProxy.php
index 750cf0a..afe800b 100644
--- a/core/lib/Drupal/Core/Session/AccountProxy.php
+++ b/core/lib/Drupal/Core/Session/AccountProxy.php
@@ -55,13 +55,6 @@ public function setAccount(AccountInterface $account) {
   /**
    * {@inheritdoc}
    */
-  public function hasAccount() {
-    return isset($this->account);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
   public function getAccount() {
     if (!isset($this->account)) {
       if ($this->id) {
diff --git a/core/lib/Drupal/Core/Session/AccountProxyInterface.php b/core/lib/Drupal/Core/Session/AccountProxyInterface.php
index b7525aa..378b89d 100644
--- a/core/lib/Drupal/Core/Session/AccountProxyInterface.php
+++ b/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 getAccount();
    */
   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 --git a/core/modules/filter/src/Tests/FilterFormTest.php b/core/modules/filter/src/Tests/FilterFormTest.php
index 9d0641e..5273346 100644
--- a/core/modules/filter/src/Tests/FilterFormTest.php
+++ b/core/modules/filter/src/Tests/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()->listExtensions();
   }
 
   /**
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 2c2c6b6..2dcbaf0 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -986,6 +986,7 @@ function system_get_info($type, $name = NULL) {
  *   function is no longer used in Drupal core.
  */
 function _system_rebuild_module_data_ensure_required($module, &$modules) {
+  @trigger_error("_system_rebuild_module_data_ensure_required() is deprecated in Drupal 8.4.0 and will be removed before Drupal 9.0.0. This function is no longer used in Drupal core. See https://www.drupal.org/node/2709919", E_USER_DEPRECATED);
   if (!empty($module->info['required'])) {
     foreach ($module->info['dependencies'] as $dependency) {
       $dependency_name = ModuleHandler::parseDependency($dependency)['name'];
@@ -1007,7 +1008,8 @@ function _system_rebuild_module_data_ensure_required($module, &$modules) {
  *
  * @deprecated in Drupal 8.4.x and will be removed before Drupal 9.0.0.
  *   Use \Drupal::service('extension.list.module')->reset()->listExtensions()
- *   instead. Note: You probably don't need the reset() method.
+ *   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()->listExtensions(). See https://www.drupal.org/node/2709919", E_USER_DEPRECATED);
@@ -1019,11 +1021,8 @@ function _system_rebuild_module_data() {
  *
  * @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()->listExtensions(). See https://www.drupal.org/node/2709919", E_USER_DEPRECATED);
   return \Drupal::service('extension.list.module')->reset()->listExtensions();
 }
 
diff --git a/core/modules/update/update.module b/core/modules/update/update.module
index 32b5b8c..c43d1af 100644
--- a/core/modules/update/update.module
+++ b/core/modules/update/update.module
@@ -607,7 +607,7 @@ function template_preprocess_update_last_check(&$variables) {
  * version of Drupal core.
  *
  * @see \Drupal\Core\Extension\ExtensionDiscovery
- * @see _system_rebuild_module_data()
+ * @see \Drupal\Core\Extension\ModuleExtensionList
  */
 function update_verify_update_archive($project, $archive_file, $directory) {
   $errors = [];
diff --git a/core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php b/core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php
index 3e4da51..9f57608 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php
@@ -21,6 +21,16 @@
 class ExtensionListTest extends UnitTestCase {
 
   /**
+   * @covers ::extensionExists
+   */
+  public function testExtensionExists() {
+    $test_extension_list = $this->setupTestExtensionList();
+
+    $this->assertTrue($test_extension_list->extensionExists('test_name'));
+    $this->assertFalse($test_extension_list->extensionExists('does_not_exist'));
+  }
+
+  /**
    * @covers ::getName
    */
   public function testGetNameWithNonExistingExtension() {
@@ -31,7 +41,7 @@ public function testGetNameWithNonExistingExtension() {
     $extension_discovery->scan('test_extension')->willReturn([]);
     $test_extension_list->setExtensionDiscovery($extension_discovery->reveal());
 
-    $this->setExpectedException(\InvalidArgumentException::class);
+    $this->setExpectedException(\InvalidArgumentException::class, 'The test_extension test_name does not exist.');
     $test_extension_list->getName('test_name');
   }
 
@@ -55,7 +65,7 @@ public function testGetExtensionWithNonExistingExtension() {
     $extension_discovery->scan('test_extension')->willReturn([]);
     $test_extension_list->setExtensionDiscovery($extension_discovery->reveal());
 
-    $this->setExpectedException(\InvalidArgumentException::class);
+    $this->setExpectedException(\InvalidArgumentException::class, 'The test_extension test_name does not exist.');
     $test_extension_list->getExtension('test_name');
   }
 
@@ -99,6 +109,15 @@ public function testGetExtensionInfo() {
   }
 
   /**
+   * @covers ::getExtensionInfo
+   */
+  public function testGetExtensionInfoException() {
+    $test_extension_list = $this->setupTestExtensionList();
+    $this->setExpectedException(\InvalidArgumentException::class, 'The test_extension test_name does not exist or is not installed.');
+    $test_extension_list->getExtensionInfo('test_name');
+  }
+
+  /**
    * @covers ::getAllAvailableInfo
    */
   public function testGetAllAvailableInfo() {
@@ -151,6 +170,14 @@ public function testGetPathname() {
     $this->assertEquals('vfs://drupal_root/example/test_name/test_name.info.yml', $pathname);
   }
 
+  /**
+   * @covers ::getPathname
+   */
+  public function testGetPathnameException() {
+    $test_extension_list = $this->setupTestExtensionList();
+    $this->setExpectedException(\InvalidArgumentException::class, 'The test_extension does_not_exist does not exist.');
+    $test_extension_list->getPathname('does_not_exist');
+  }
 
   /**
    * @covers ::setPathname
diff --git a/core/tests/Drupal/Tests/Core/Session/AccountProxyTest.php b/core/tests/Drupal/Tests/Core/Session/AccountProxyTest.php
index f17858cf..0e575a0 100644
--- a/core/tests/Drupal/Tests/Core/Session/AccountProxyTest.php
+++ b/core/tests/Drupal/Tests/Core/Session/AccountProxyTest.php
@@ -42,17 +42,6 @@ public function testSetInitialAccountIdException() {
     $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;