diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 23d63bc..26a75f6 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -86,9 +86,7 @@
  * @see http://php.net/manual/reserved.variables.server.php
  * @see http://php.net/manual/function.time.php
  */
-if (!defined('REQUEST_TIME')) {
-  define('REQUEST_TIME', (int) $_SERVER['REQUEST_TIME']);
-}
+define('REQUEST_TIME', (int) $_SERVER['REQUEST_TIME']);
 
 /**
  * Regular expression to match PHP function names.
@@ -116,9 +114,7 @@
  *
  * This strips two levels of directories off the current directory.
  */
-if (!defined('DRUPAL_ROOT')) {
-  define('DRUPAL_ROOT', dirname(dirname(__DIR__)));
-}
+define('DRUPAL_ROOT', dirname(dirname(__DIR__)));
 
 /**
  * Returns the appropriate configuration directory.
diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 791c660..8caadaf 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -148,13 +148,16 @@ class DrupalKernel implements DrupalKernelInterface, TerminableInterface {
   protected $serviceYamls;
 
   /**
-   * List of discovered service provider class names.
+   * List of discovered service provider class names or objects.
    *
    * This is a nested array whose top-level keys are 'app' and 'site', denoting
    * the origin of a service provider. Site-specific providers have to be
    * collected separately, because they need to be processed last, so as to be
    * able to override services from application service providers.
    *
+   * Allowing objects is for example used for allowing
+   * \Drupal\Tests\KernelTestBase to register itself as service provider.
+   *
    * @var array
    */
   protected $serviceProviderClasses;
diff --git a/core/modules/system/src/Tests/Extension/ModuleHandlerTest.php b/core/modules/system/src/Tests/Extension/ModuleHandlerTest.php
index 87b3af4..dab71d4 100644
--- a/core/modules/system/src/Tests/Extension/ModuleHandlerTest.php
+++ b/core/modules/system/src/Tests/Extension/ModuleHandlerTest.php
@@ -24,7 +24,7 @@ class ModuleHandlerTest extends KernelTestBase {
   protected function setUp() {
     parent::setUp();
 
-    // @fixme ModuleInstaller calls system_rebuild_module_data which is part of
+    // @todo ModuleInstaller calls system_rebuild_module_data which is part of
     // system.module, see https://www.drupal.org/node/2208429
     include_once $this->root . '/core/modules/system/system.module';
   }
diff --git a/core/tests/Drupal/Tests/KernelTestBase.php b/core/tests/Drupal/Tests/KernelTestBase.php
index 9984a6f..1266189 100644
--- a/core/tests/Drupal/Tests/KernelTestBase.php
+++ b/core/tests/Drupal/Tests/KernelTestBase.php
@@ -63,7 +63,7 @@
   /**
    * Flag to reset any manipulations to global variables between tests.
    *
-   * TRUE by default, but MUST be TRUE for kernel tests.
+   * Don't change the value, unless you really know what you are doing.
    *
    * @var bool
    */
diff --git a/core/tests/Drupal/Tests/KernelTestBaseTest.php b/core/tests/Drupal/Tests/KernelTestBaseTest.php
index b68bbb5..6f228dd 100644
--- a/core/tests/Drupal/Tests/KernelTestBaseTest.php
+++ b/core/tests/Drupal/Tests/KernelTestBaseTest.php
@@ -104,7 +104,7 @@ public function testSetUp() {
   public function testSetUpDoesNotLeak() {
     $this->assertArrayNotHasKey('destroy-me', $GLOBALS);
 
-    // We store teh active configuration in the DB.
+    // We store the active configuration in the DB.
     $expected = ['config' => 'config'];
     $schema = $this->container->get('database')->schema();
     $this->assertEquals($expected, $schema->findTables('%'));