diff -u b/core/modules/language/src/Tests/EntityUrlLanguageTest.php b/core/modules/language/src/Tests/EntityUrlLanguageTest.php
--- b/core/modules/language/src/Tests/EntityUrlLanguageTest.php
+++ b/core/modules/language/src/Tests/EntityUrlLanguageTest.php
@@ -12,6 +12,9 @@
 use Drupal\language\Entity\ConfigurableLanguage;
 use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationContentEntity;
 use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationUrl;
+use Symfony\Cmf\Component\Routing\RouteObjectInterface;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\Routing\Route;
 
 /**
  * Tests the language of entity URLs.
@@ -49,8 +52,7 @@
     $config = $this->config('language.negotiation');
     $config->set('url.prefixes', ['en' => 'en', 'es' => 'es', 'fr' => 'fr'])
       ->save();
-
-    $this->rebuildContainer();
+    $this->kernel->rebuildContainer();
 
     $this->createTranslatableEntity();
   }
@@ -83,7 +85,7 @@
 
     // In order to reflect the changes for a multilingual site in the container
     // we have to rebuild it.
-    $this->rebuildContainer();
+    $this->kernel->rebuildContainer();
 
     // Without being on an content entity route the default entity url tests
     // should still pass.
@@ -111,7 +113,7 @@
 
     // In order to reflect the changes for a multilingual site in the container
     // we have to rebuild it.
-    $this->rebuildContainer();
+    $this->kernel->rebuildContainer();
 
     // The default entity url tests should pass again with the current
     // configuration.
@@ -130,2 +132,18 @@
 
+  /**
+   * Sets the current request to a specific path with the corresponding route.
+   *
+   * @param string $path
+   *   The path for which the current request should be created.
+   * @param string $route_name
+   *   The route name for which the route object for the request should be
+   *   created.
+   */
+  protected function setCurrentRequestForRoute($path, $route_name) {
+    $request = Request::create($path);
+    $request->attributes->set(RouteObjectInterface::ROUTE_NAME, $route_name);
+    $request->attributes->set(RouteObjectInterface::ROUTE_OBJECT, new Route($path));
+    $this->container->get('request_stack')->push($request);
+  }
+
 }
diff -u b/core/modules/language/src/Tests/LanguageNegotiationContentEntityTest.php b/core/modules/language/src/Tests/LanguageNegotiationContentEntityTest.php
--- b/core/modules/language/src/Tests/LanguageNegotiationContentEntityTest.php
+++ b/core/modules/language/src/Tests/LanguageNegotiationContentEntityTest.php
@@ -14,6 +14,9 @@
 use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationContentEntity;
 use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationUrl;
 use Drupal\simpletest\WebTestBase;
+use Symfony\Cmf\Component\Routing\RouteObjectInterface;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\Routing\Route;
 
 /**
  * Tests language negotiation with the language negotiator content entity.
@@ -161,2 +164,18 @@
   }
+
+  /**
+   * Sets the current request to a specific path with the corresponding route.
+   *
+   * @param string $path
+   *   The path for which the current request should be created.
+   * @param string $route_name
+   *   The route name for which the route object for the request should be
+   *   created.
+   */
+  protected function setCurrentRequestForRoute($path, $route_name) {
+    $request = Request::create($path);
+    $request->attributes->set(RouteObjectInterface::ROUTE_NAME, $route_name);
+    $request->attributes->set(RouteObjectInterface::ROUTE_OBJECT, new Route($path));
+    $this->container->get('request_stack')->push($request);
+  }
 }
reverted:
--- b/core/modules/simpletest/src/TestBase.php
+++ a/core/modules/simpletest/src/TestBase.php
@@ -19,9 +19,6 @@
 use Drupal\Core\Site\Settings;
 use Drupal\Core\StreamWrapper\PublicStream;
 use Drupal\Core\Utility\Error;
-use Symfony\Cmf\Component\Routing\RouteObjectInterface;
-use Symfony\Component\HttpFoundation\Request;
-use Symfony\Component\Routing\Route;
 
 /**
  * Base class for Drupal tests.
@@ -1589,28 +1586,4 @@
     return $this->tempFilesDirectory;
   }
 
-  /**
-   * Force a container rebuild.
-   */
-  protected function rebuildContainer() {
-    // Rebuild the kernel and bring it back to a fully bootstrapped state.
-    $this->container = $this->kernel->rebuildContainer();
-  }
-
-  /**
-   * Sets the current request to a specific path with the corresponding route.
-   *
-   * @param string $path
-   *   The path for which the current request should be created.
-   * @param string $route_name
-   *   The route name for which the route object for the request should be
-   *   created.
-   */
-  protected function setCurrentRequestForRoute($path, $route_name) {
-    $request = Request::create($path);
-    $request->attributes->set(RouteObjectInterface::ROUTE_NAME, $route_name);
-    $request->attributes->set(RouteObjectInterface::ROUTE_OBJECT, new Route($path));
-    \Drupal::requestStack()->push($request);
-  }
-
 }
reverted:
--- b/core/modules/simpletest/src/WebTestBase.php
+++ a/core/modules/simpletest/src/WebTestBase.php
@@ -1173,7 +1173,8 @@
    *   enabled modules to be immediately available in the same request.
    */
   protected function rebuildContainer() {
+    // Rebuild the kernel and bring it back to a fully bootstrapped state.
+    $this->container = $this->kernel->rebuildContainer();
-    parent::rebuildContainer();
 
     // Make sure the url generator has a request object, otherwise calls to
     // $this->drupalGet() will fail.