diff --git a/core/lib/Drupal/Component/Utility/Html.php b/core/lib/Drupal/Component/Utility/Html.php
index cde2a74..b10f203 100644
--- a/core/lib/Drupal/Component/Utility/Html.php
+++ b/core/lib/Drupal/Component/Utility/Html.php
@@ -35,7 +35,7 @@ class Html {
   protected static $seenIds;
 
   /**
-   * Contains if the current request was send via AJAX.
+   * Contains if the current request was sent via AJAX.
    *
    * @var bool
    */
diff --git a/core/lib/Drupal/Core/EventSubscriber/AjaxSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/AjaxSubscriber.php
index ab2ab8b..83262f8 100644
--- a/core/lib/Drupal/Core/EventSubscriber/AjaxSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/AjaxSubscriber.php
@@ -20,13 +20,19 @@
 class AjaxSubscriber implements EventSubscriberInterface {
 
   /**
+   * Request parameter to indicate that a request is a drupal ajax request.
+   */
+  const AJAX_REQUEST_PARAMETER = '_drupal_ajax';
+
+  /**
    * Sets the AJAX parameter from the current request.
    *
    * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
    *   The response event, which contains the current request.
    */
   public function onRequest(GetResponseEvent $event) {
-    if ($event->getRequest()->request->get('ajax', '') == 1) {
+    // Ensure that ajax=1 is set.
+    if ($event->getRequest()->request->get(static::AJAX_REQUEST_PARAMETER)) {
       Html::setIsAjax(TRUE);
     }
   }
diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index f47eb27..8f2b8cd 100644
--- a/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -377,6 +377,11 @@
   Drupal.ajax.WRAPPER_FORMAT = '_wrapper_format';
 
   /**
+   * Request parameter to indicate that a request is a drupal ajax request.
+   */
+   DRupal.ajax.AJAX_REQUEST_PARAMETER = '_drupal_ajax';
+
+  /**
    * Execute the ajax request.
    *
    * Allows developers to execute an Ajax request manually without specifying
@@ -491,8 +496,8 @@
       Drupal.detachBehaviors(this.$form.get(0), settings, 'serialize');
     }
 
-    // Set that this is an drupal ajax request.
-    options.data.ajax = 1;
+    // Inform Drupal that this is an AJAX request.
+    options.data[Drupal.ajax.AJAX_REQUEST_PARAMETER] = 1;
 
     // Allow Drupal to return new JavaScript and CSS files to load without
     // returning the ones already loaded.
diff --git a/core/modules/file/src/Tests/FileFieldWidgetTest.php b/core/modules/file/src/Tests/FileFieldWidgetTest.php
index 4566d27..10f2cad 100644
--- a/core/modules/file/src/Tests/FileFieldWidgetTest.php
+++ b/core/modules/file/src/Tests/FileFieldWidgetTest.php
@@ -9,7 +9,6 @@
 
 use Drupal\comment\Entity\Comment;
 use Drupal\comment\Tests\CommentTestTrait;
-use Drupal\Component\Utility\Html;
 use Drupal\field\Entity\FieldConfig;
 use Drupal\field_ui\Tests\FieldUiTestTrait;
 use Drupal\user\RoleInterface;