diff --git a/core/modules/simpletest/src/TestBase.php b/core/modules/simpletest/src/TestBase.php
index 01a0cfb..ac2deac 100644
--- a/core/modules/simpletest/src/TestBase.php
+++ b/core/modules/simpletest/src/TestBase.php
@@ -1071,7 +1071,7 @@ private function prepareEnvironment() {
     // Reset statics.
     drupal_static_reset();
 
-    // Reset and create a new service container.
+    // Ensure there is no service container.
     $this->container = NULL;
     \Drupal::setContainer(NULL);
 
@@ -1209,7 +1209,6 @@ private function restoreEnvironment() {
     $callbacks = $this->originalShutdownCallbacks;
 
     // Restore original user session.
-    \Drupal::getContainer()->set('current_user', $this->originalUser);
     \Drupal::service('session_manager')->enable();
   }
 
diff --git a/core/modules/system/src/Tests/Common/HtmlIdentifierUnitTest.php b/core/modules/system/src/Tests/Common/HtmlIdentifierUnitTest.php
index 3d076ce..cdfc45a 100644
--- a/core/modules/system/src/Tests/Common/HtmlIdentifierUnitTest.php
+++ b/core/modules/system/src/Tests/Common/HtmlIdentifierUnitTest.php
@@ -7,12 +7,12 @@
 
 namespace Drupal\system\Tests\Common;
 
-use Drupal\simpletest\DrupalUnitTestBase;
+use Drupal\simpletest\KernelTestBase;
 
 /**
  * Tests cleaning HTML identifiers.
  */
-class HtmlIdentifierUnitTest extends DrupalUnitTestBase {
+class HtmlIdentifierUnitTest extends KernelTestBase {
   public static function getInfo() {
     return array(
       'name' => 'HTML identifiers',
diff --git a/core/modules/system/src/Tests/Common/SizeUnitTest.php b/core/modules/system/src/Tests/Common/SizeUnitTest.php
index c2b9959..680f010 100644
--- a/core/modules/system/src/Tests/Common/SizeUnitTest.php
+++ b/core/modules/system/src/Tests/Common/SizeUnitTest.php
@@ -8,12 +8,12 @@
 namespace Drupal\system\Tests\Common;
 
 use Drupal\Component\Utility\Bytes;
-use Drupal\simpletest\DrupalUnitTestBase;
+use Drupal\simpletest\KernelTestBase;
 
 /**
  * Tests file size parsing and formatting functions.
  */
-class SizeUnitTest extends DrupalUnitTestBase {
+class SizeUnitTest extends KernelTestBase {
   protected $exact_test_cases;
   protected $rounded_test_cases;
 
diff --git a/core/modules/system/src/Tests/Common/TableSortExtenderUnitTest.php b/core/modules/system/src/Tests/Common/TableSortExtenderUnitTest.php
index 9f93c44..ebc6269 100644
--- a/core/modules/system/src/Tests/Common/TableSortExtenderUnitTest.php
+++ b/core/modules/system/src/Tests/Common/TableSortExtenderUnitTest.php
@@ -7,13 +7,13 @@
 
 namespace Drupal\system\Tests\Common;
 
-use Drupal\simpletest\DrupalUnitTestBase;
+use Drupal\simpletest\KernelTestBase;
 use Symfony\Component\HttpFoundation\Request;
 
 /**
  * Tests table sorting features implemented in tablesort.inc.
  */
-class TableSortExtenderUnitTest extends DrupalUnitTestBase {
+class TableSortExtenderUnitTest extends KernelTestBase {
 
   public static function getInfo() {
     return array(
diff --git a/core/modules/system/src/Tests/Plugin/Discovery/DiscoveryTestBase.php b/core/modules/system/src/Tests/Plugin/Discovery/DiscoveryTestBase.php
index a3ab7b3..3015d14 100644
--- a/core/modules/system/src/Tests/Plugin/Discovery/DiscoveryTestBase.php
+++ b/core/modules/system/src/Tests/Plugin/Discovery/DiscoveryTestBase.php
@@ -7,12 +7,12 @@
 
 namespace Drupal\system\Tests\Plugin\Discovery;
 
-use Drupal\simpletest\DrupalUnitTestBase;
+use Drupal\simpletest\KernelTestBase;
 
 /**
  * Base class for plugin discovery tests.
  */
-abstract class DiscoveryTestBase extends DrupalUnitTestBase {
+abstract class DiscoveryTestBase extends KernelTestBase {
 
   /**
    * The discovery component to test.
diff --git a/core/modules/system/src/Tests/Plugin/PluginTestBase.php b/core/modules/system/src/Tests/Plugin/PluginTestBase.php
index fdb4b2d..72b0ffe 100644
--- a/core/modules/system/src/Tests/Plugin/PluginTestBase.php
+++ b/core/modules/system/src/Tests/Plugin/PluginTestBase.php
@@ -7,7 +7,7 @@
 
 namespace Drupal\system\Tests\Plugin;
 
-use Drupal\simpletest\DrupalUnitTestBase;
+use Drupal\simpletest\KernelTestBase;
 use Drupal\plugin_test\Plugin\TestPluginManager;
 use Drupal\plugin_test\Plugin\MockBlockManager;
 use Drupal\plugin_test\Plugin\DefaultsTestPluginManager;
@@ -17,7 +17,7 @@
 /**
  * Base class for Plugin API unit tests.
  */
-abstract class PluginTestBase extends DrupalUnitTestBase {
+abstract class PluginTestBase extends KernelTestBase {
 
   /**
    * Modules to enable.