diff --git a/core/lib/Drupal/Core/EventSubscriber/AcceptNegotiation406.php b/core/lib/Drupal/Core/EventSubscriber/AcceptNegotiation406.php
index 72683e1..468ab9f 100644
--- a/core/lib/Drupal/Core/EventSubscriber/AcceptNegotiation406.php
+++ b/core/lib/Drupal/Core/EventSubscriber/AcceptNegotiation406.php
@@ -8,7 +8,6 @@
 namespace Drupal\Core\EventSubscriber;
 
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
-use Symfony\Component\HttpFoundation\JsonResponse;
 use Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent;
 use Symfony\Component\HttpKernel\Exception\NotAcceptableHttpException;
 use Symfony\Component\HttpKernel\KernelEvents;
diff --git a/core/lib/Drupal/Core/Render/Element/RenderElement.php b/core/lib/Drupal/Core/Render/Element/RenderElement.php
index b9de164..87b9f02 100644
--- a/core/lib/Drupal/Core/Render/Element/RenderElement.php
+++ b/core/lib/Drupal/Core/Render/Element/RenderElement.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\Core\Render\Element;
 
-use Drupal\Core\EventSubscriber\MainContentViewSubscriber;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Plugin\PluginBase;
 use Drupal\Core\Render\Element;
@@ -241,7 +240,7 @@ public static function preRenderAjaxForm($element) {
       $settings += array(
         'url' => isset($settings['callback']) ? Url::fromRoute('system.ajax') : NULL,
         'options' => array(),
-        MainContentViewSubscriber::WRAPPER_ENVELOPE => 'ajax',
+        'dialogType' => 'ajax',
       );
 
       // @todo Legacy support. Remove in Drupal 8.
diff --git a/core/modules/system/tests/modules/ajax_test/src/Controller/AjaxTestController.php b/core/modules/system/tests/modules/ajax_test/src/Controller/AjaxTestController.php
index 364a36f..ff4859d 100644
--- a/core/modules/system/tests/modules/ajax_test/src/Controller/AjaxTestController.php
+++ b/core/modules/system/tests/modules/ajax_test/src/Controller/AjaxTestController.php
@@ -120,6 +120,7 @@ public function dialog() {
       '#url' => Url::fromRoute('ajax_test.dialog_contents'),
       '#attributes' => array(
         'class' => array('use-ajax'),
+        'data-dialog-type' => 'modal',
       ),
     );