diff --git a/core/modules/simpletest/src/InstallerTestBase.php b/core/tests/Drupal/FunctionalTests/Installer/InstallerTestBase.php
index ae7c734dbd..55569a28d9 100644
--- a/core/modules/simpletest/src/InstallerTestBase.php
+++ b/core/tests/Drupal/FunctionalTests/Installer/InstallerTestBase.php
@@ -1,11 +1,16 @@
 <?php
 
-namespace Drupal\simpletest;
+namespace Drupal\FunctionalTests\Installer;
+
+require_once __DIR__ . '/../../../../includes/bootstrap.inc';
 
 use Drupal\Core\DrupalKernel;
 use Drupal\Core\Language\Language;
 use Drupal\Core\Session\UserSession;
 use Drupal\Core\Site\Settings;
+use Drupal\Core\Test\HttpClientMiddleware\TestHttpClientMiddleware;
+use Drupal\Tests\BrowserTestBase;
+use GuzzleHttp\HandlerStack;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\Reference;
 use Symfony\Component\HttpFoundation\Request;
@@ -14,7 +19,7 @@
 /**
  * Base class for testing the interactive installer.
  */
-abstract class InstallerTestBase extends WebTestBase {
+abstract class InstallerTestBase extends BrowserTestBase {
 
   /**
    * Custom settings.php values to write for a test run.
@@ -42,7 +47,7 @@
   /**
    * Additional parameters to use for installer screens.
    *
-   * @see WebTestBase::installParameters()
+   * @see FunctionalTestSetupTrait::installParameters()
    *
    * @var array
    */
@@ -72,6 +77,13 @@
   protected function setUp() {
     $this->isInstalled = FALSE;
 
+    $this->setupBaseUrl();
+
+    $this->prepareDatabasePrefix();
+
+    // Install Drupal test site.
+    $this->prepareEnvironment();
+
     // Define information about the user 1 account.
     $this->rootUser = new UserSession([
       'uid' => 1,
@@ -88,12 +100,13 @@ protected function setUp() {
       $this->writeSettings($this->settings);
     }
 
-    // Note that WebTestBase::installParameters() returns form input values
-    // suitable for a programmed \Drupal::formBuilder()->submitForm().
-    // @see WebTestBase::translatePostValues()
+    // Note that FunctionalTestSetupTrait::installParameters() returns form
+    // input values suitable for a programmed
+    // \Drupal::formBuilder()->submitForm().
+    // @see InstallerTestBase::translatePostValues()
     $this->parameters = $this->installParameters();
 
-    // Set up a minimal container (required by WebTestBase). Set cookie and
+    // Set up a minimal container (required by BrowserTestBase). Set cookie and
     // server information so that XDebug works.
     // @see install_begin_request()
     $request = Request::create($GLOBALS['base_url'] . '/core/install.php', 'GET', [], $_COOKIE, [], $_SERVER);
@@ -110,10 +123,25 @@ protected function setUp() {
     $this->container
       ->register('string_translation', 'Drupal\Core\StringTranslation\TranslationManager')
       ->addArgument(new Reference('language.default'));
+    $this->container
+      ->register('http_client', 'GuzzleHttp\Client')
+      ->setFactory('http_client_factory:fromOptions');
+    $this->container
+      ->register('http_client_factory', 'Drupal\Core\Http\ClientFactory')
+      ->setArguments([new Reference('http_handler_stack')]);
+    $handler_stack = HandlerStack::create();
+    $test_http_client_middleware = new TestHttpClientMiddleware();
+    $handler_stack->push($test_http_client_middleware(), 'test.http_client.middleware');
+    $this->container
+      ->set('http_handler_stack', $handler_stack);
+
     $this->container
       ->set('app.root', DRUPAL_ROOT);
     \Drupal::setContainer($this->container);
 
+    // Setup Mink.
+    $this->initMink();
+
     $this->visitInstaller();
 
     // Select language.
@@ -147,7 +175,7 @@ protected function setUp() {
       // from the site directory. To allow drupal_generate_test_ua() to write
       // a file containing the private key for drupal_valid_test_ua(), the site
       // directory has to be writable.
-      // WebTestBase::tearDown() will delete the entire test site directory.
+      // BrowserTestBase::tearDown() will delete the entire test site directory.
       // Not using File API; a potential error must trigger a PHP warning.
       chmod($this->container->get('app.root') . '/' . $this->siteDirectory, 0777);
       $this->kernel = DrupalKernel::createFromRequest($request, $class_loader, 'prod', FALSE);
@@ -163,6 +191,14 @@ protected function setUp() {
     }
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function initFrontPage() {
+    // We don't want to visit the front page with the installer when
+    // initializing Mink, so we do nothing here.
+  }
+
   /**
    * Visits the interactive installer.
    */
@@ -226,11 +262,33 @@ protected function setUpSite() {
     $this->isInstalled = TRUE;
   }
 
+  /**
+   * Transforms a nested array into a flat array suitable for drupalPostForm().
+   *
+   * @param array $values
+   *   A multi-dimensional form values array to convert.
+   *
+   * @return array
+   *   The flattened $edit array suitable for BrowserTestBase::drupalPostForm().
+   */
+  protected function translatePostValues(array $values) {
+    $edit = [];
+    // The easiest and most straightforward way to translate values suitable for
+    // BrowserTestBase::drupalPostForm() is to actually build the POST data
+    // string and convert the resulting key/value pairs back into a flat array.
+    $query = http_build_query($values);
+    foreach (explode('&', $query) as $item) {
+      list($key, $value) = explode('=', $item);
+      $edit[urldecode($key)] = urldecode($value);
+    }
+    return $edit;
+  }
+
   /**
    * {@inheritdoc}
    *
-   * WebTestBase::refreshVariables() tries to operate on persistent storage,
-   * which is only available after the installer completed.
+   * FunctionalTestSetupTrait::refreshVariables() tries to operate on persistent
+   * storage, which is only available after the installer completed.
    */
   protected function refreshVariables() {
     if ($this->isInstalled) {
@@ -272,10 +330,11 @@ protected function continueOnExpectedWarnings($expected_warnings = []) {
     // Confirm that there are only the expected warnings.
     $warnings = [];
     foreach ($warning_elements as $warning) {
-      $warnings[] = trim((string) $warning);
+      $warnings[] = trim($warning->getText());
     }
-    $this->assertEqual($expected_warnings, $warnings);
+    $this->assertEquals($expected_warnings, $warnings);
     $this->clickLink('continue anyway');
+    $this->checkForMetaRefresh();
   }
 
 }