diff -u b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
--- b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -668,10 +668,10 @@
     // restores the original container.
     // @see Drupal\Core\DrupalKernel::initializeContainer()
     $this->kernel = new DrupalKernel('testing', FALSE);
-    // While normally the kernel gets booted on demand in
-    // Symfony\Component\HttpKernel\handle() this kernel is only used to
-    // create a new dependency injection container and so it needs manual
-    // booting.
+    // Booting the kernel is necessary to initialize the new DIC. While
+    // normally the kernel gets booted on demand in
+    // Symfony\Component\HttpKernel\handle(), this kernel needs manual booting
+    // as it is not used to handle a request.
     $this->kernel->boot();
 
     // Reset/rebuild all data structures after enabling the modules.