diff --git a/core/modules/simpletest/src/InstallerTestBase.php b/core/modules/simpletest/src/InstallerTestBase.php
index e735ce3..4befd78 100644
--- a/core/modules/simpletest/src/InstallerTestBase.php
+++ b/core/modules/simpletest/src/InstallerTestBase.php
@@ -7,7 +7,10 @@
 
 namespace Drupal\simpletest;
 
+use Drupal\Core\Language\Language;
 use Drupal\Core\Session\UserSession;
+use Symfony\Component\DependencyInjection\Reference;
+use Symfony\Component\HttpFoundation\Request;
 
 /**
  * Base class for testing the interactive installer.
@@ -91,6 +94,22 @@ protected function setUp() {
     // @see WebTestBase::translatePostValues()
     $this->parameters = $this->installParameters();
 
+    // WebTestBase::curlExec calls into format_size and that depends on the
+    // string_translation service. Because the parent constructor is disabled
+    // entirely, it is necessary to set up the string_translation service here
+    // before calling WebTestBase::drupalGet()
+    $this->container->setParameter('language.default_values', Language::$defaultValues);
+    $this->container->register('language.default', 'Drupal\Core\Language\LanguageDefault')
+      ->addArgument('%language.default_values%');
+    $this->container->register('language_manager', 'Drupal\Core\Language\LanguageManager')
+      ->addArgument(new Reference('language.default'));
+    $this->container->register('string_translation', 'Drupal\Core\StringTranslation\TranslationManager')
+      ->addArgument(new Reference('language_manager'));
+
+    // Make sure the container has a request service, otherwise calls to
+    // $this->drupalGet() will fail.
+    $this->container->set('request', Request::createFromGlobals());
+
     $this->drupalGet($GLOBALS['base_url'] . '/core/install.php');
 
     // Select language.
diff --git a/core/modules/simpletest/src/TestBase.php b/core/modules/simpletest/src/TestBase.php
index 5938426..48f273f 100644
--- a/core/modules/simpletest/src/TestBase.php
+++ b/core/modules/simpletest/src/TestBase.php
@@ -16,14 +16,11 @@
 use Drupal\Core\Database\ConnectionNotDefinedException;
 use Drupal\Core\Config\StorageInterface;
 use Drupal\Core\DrupalKernel;
-use Drupal\Core\Language\Language;
 use Drupal\Core\Session\AccountProxy;
 use Drupal\Core\Session\AnonymousUserSession;
 use Drupal\Core\Site\Settings;
 use Drupal\Core\StreamWrapper\PublicStream;
 use Drupal\Core\Utility\Error;
-use Symfony\Component\HttpFoundation\Request;
-use Symfony\Component\DependencyInjection\Reference;
 
 /**
  * Base class for Drupal tests.
diff --git a/core/modules/system/src/Tests/Plugin/Discovery/DiscoveryTestBase.php b/core/modules/system/src/Tests/Plugin/Discovery/DiscoveryTestBase.php
index 9d0be54..a3ab7b3 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\UnitTestBase;
+use Drupal\simpletest\DrupalUnitTestBase;
 
 /**
  * Base class for plugin discovery tests.
  */
-abstract class DiscoveryTestBase extends UnitTestBase {
+abstract class DiscoveryTestBase extends DrupalUnitTestBase {
 
   /**
    * 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 3a92dbd..bd97bba 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\UnitTestBase;
+use Drupal\simpletest\DrupalUnitTestBase;
 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 UnitTestBase {
+abstract class PluginTestBase extends DrupalUnitTestBase {
   protected $testPluginManager;
   protected $testPluginExpectedDefinitions;
   protected $mockBlockManager;