diff --git a/core/includes/ajax.inc b/core/includes/ajax.inc
index 965913d..298f0ec 100644
--- a/core/includes/ajax.inc
+++ b/core/includes/ajax.inc
@@ -1153,17 +1153,3 @@ function ajax_command_add_css($styles) {
     'data' => $styles,
   );
 }
-
-/**
- * Helper function that checks whether this is an AJAX request.
- *
- * @return bool
- *   Return TRUE if the current request is asking for ajax content, else FALSE.
- */
-function ajax_is_ajax_request() {
-  $container = drupal_container();
-  if ($container->has('content_negotiation') && $container->isScopeActive('request')) {
-    return $container->get('content_negotiation')->getContentType($container->get('request')) == 'ajax';
-  }
-  return FALSE;
-}
diff --git a/modules/dialog_example/dialog_example.module b/modules/dialog_example/dialog_example.module
index 9ad1cfa..ce81e47 100644
--- a/modules/dialog_example/dialog_example.module
+++ b/modules/dialog_example/dialog_example.module
@@ -113,7 +113,7 @@ function dialog_example_form($form_type, $page_type = 'nojs') {
       break;
   }
 
-  if (ajax_is_ajax_request()) {
+  if (Drupal::service('request')->isXmlHttpRequest()) {
     $response = new AjaxResponse();
     $response->addCommand(new openModalDialogCommand('Title', drupal_render($output)));
     return $response;
@@ -173,7 +173,7 @@ function dialog_example_complex_form() {
     '#weight' => -10,
   );
 
-  if (ajax_is_ajax_request()) {
+  if (Drupal::service('request')->isXmlHttpRequest()) {
     $form['actions']['okayish']['#ajax'] = array(
       'callback' => 'dialog_example_complex_okayish',
     );