diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 33d32f8..6bd911b 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -252,9 +252,9 @@ class DrupalKernel implements DrupalKernelInterface, TerminableInterface {
    * @param bool $allow_dumping
    *   (optional) FALSE to stop the container from being written to or read
    *   from disk. Defaults to TRUE.
-   * @param string $app_root
-   *   (optional) The path to the application root as a string. If omitted or
-   *   NULL, the application root will be computed.
+   * @param string|null $app_root
+   *   (optional) The path to the application root as a string, or NULL. If
+   *   NULL, the application root will be computed. Defaults to NULL.
    *
    * @return static
    *
@@ -280,9 +280,9 @@ public static function createFromRequest(Request $request, $class_loader, $envir
    * @param bool $allow_dumping
    *   (optional) FALSE to stop the container from being written to or read
    *   from disk. Defaults to TRUE.
-   * @param string $app_root
-   *   (optional) The path to the application root as a string. If omitted or
-   *   NULL, the application root will be computed.
+   * @param string|null $app_root
+   *   (optional) The path to the application root as a string, or NULL. If
+   *   NULL, the application root will be computed. Defaults to NULL.
    */
   public function __construct($environment, $class_loader, $allow_dumping = TRUE, $app_root = NULL) {
     $this->environment = $environment;
@@ -300,7 +300,7 @@ public function __construct($environment, $class_loader, $allow_dumping = TRUE,
    * @return string
    *   The application root.
    */
-  protected static function guessApplicationRoot() {
+  private static function guessApplicationRoot() {
     return dirname(dirname(substr(__DIR__, 0, -strlen(__NAMESPACE__))));
   }
 
@@ -343,9 +343,9 @@ protected static function guessApplicationRoot() {
    *   Defaults to TRUE. During initial installation, this is set to FALSE so
    *   that Drupal can detect a matching directory, then create a new
    *   settings.php file in it.
-   * @param string $app_root
-   *   (optional) The path to the application root as a string. If omitted or
-   *   NULL, the application root will be computed.
+   * @param string|null $app_root
+   *   (optional) The path to the application root as a string, or NULL. If
+   *   NULL, the application root will be computed. Defaults to NULL.
    *
    * @return string
    *   The path of the matching directory.
@@ -904,9 +904,9 @@ protected function initializeContainer() {
    * This method sets PHP environment options we want to be sure are set
    * correctly for security or just saneness.
    *
-   * @param string $app_root
-   *   (optional) The path to the application root as a string. If omitted or
-   *   NULL, the application root will be computed.
+   * @param string|null $app_root
+   *   (optional) The path to the application root as a string, or NULL. If
+   *   NULL, the application root will be computed. Defaults to NULL.
    */
   public static function bootEnvironment($app_root = NULL) {
     if (static::$isEnvironmentInitialized) {
diff --git a/core/lib/Drupal/Core/Test/TestRunnerKernel.php b/core/lib/Drupal/Core/Test/TestRunnerKernel.php
index a7cbe9f..d7402a4 100644
--- a/core/lib/Drupal/Core/Test/TestRunnerKernel.php
+++ b/core/lib/Drupal/Core/Test/TestRunnerKernel.php
@@ -28,7 +28,7 @@ public static function createFromRequest(Request $request, $class_loader, $envir
    * {@inheritdoc}
    */
   public function __construct($environment, $class_loader, $allow_dumping = FALSE, $app_root = NULL) {
-    parent::__construct($environment, $class_loader, $allow_dumping, $app_root);
+    parent::__construct($environment, $class_loader, FALSE, $app_root);
 
     // Prime the module list and corresponding Extension objects.
     // @todo Remove System module. Needed because