diff --git a/core/misc/ajax.es6.js b/core/misc/ajax.es6.js
index 3ff1a6b311..a4fefe424e 100644
--- a/core/misc/ajax.es6.js
+++ b/core/misc/ajax.es6.js
@@ -910,13 +910,13 @@
   /**
    * Provide a wrapper for the AJAX progress bar element.
    *
-   * @param {jQuery} element
+   * @param {jQuery} $element
    *   Progress bar element.
    * @return {string}
    *   The HTML markup for the progress bar.
    */
-  Drupal.theme.ajaxProgressBar = element =>
-    $('<div class="ajax-progress ajax-progress-bar"></div>').append(element);
+  Drupal.theme.ajaxProgressBar = $element =>
+    $('<div class="ajax-progress ajax-progress-bar"></div>').append($element);
 
   /**
    * Sets the progress bar progress indicator.
diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index 6261a8bd0b..030ccffbfe 100644
--- a/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -641,4 +641,4 @@ function _toConsumableArray(arr) { if (Array.isArray(arr)) { for (var i = 0, arr
       }
     }
   };
-})(jQuery, window, Drupal, drupalSettings);
+})(jQuery, window, Drupal, drupalSettings);
\ No newline at end of file
diff --git a/core/themes/stable/js/ajax.es6.js b/core/themes/stable/js/ajax.es6.js
index e69de29bb2..e33ce5e230 100644
--- a/core/themes/stable/js/ajax.es6.js
+++ b/core/themes/stable/js/ajax.es6.js
@@ -0,0 +1,17 @@
+/**
+ * @file
+ * Provides backwards compatibility layer for Ajax related markup.
+ */
+
+((Drupal) => {
+  /**
+   * Override the default ajaxProgressBar for backwards compatibility.
+   *
+   * @param {jQuery} $element
+   *   Progress bar element.
+   * @return {string}
+   *   The HTML markup for the progress bar.
+   */
+  Drupal.theme.ajaxProgressBar = $element =>
+    $element.addClass('ajax-progress ajax-progress-bar');
+})(Drupal);
diff --git a/core/themes/stable/stable.info.yml b/core/themes/stable/stable.info.yml
index ea26e95227..e5f3d29dbc 100644
--- a/core/themes/stable/stable.info.yml
+++ b/core/themes/stable/stable.info.yml
@@ -298,3 +298,7 @@ libraries-override:
       theme:
         css/views_ui.admin.theme.css: css/views_ui/views_ui.admin.theme.css
         css/views_ui.contextual.css: css/views_ui/views_ui.contextual.css
+
+libraries-extend:
+  core/drupal.ajax:
+    - stable/drupal.ajax
diff --git a/core/themes/stable/stable.libraries.yml b/core/themes/stable/stable.libraries.yml
index e69de29bb2..7c13cc6e04 100644
--- a/core/themes/stable/stable.libraries.yml
+++ b/core/themes/stable/stable.libraries.yml
@@ -0,0 +1,4 @@
+drupal.ajax:
+  version: VERSION
+  js:
+    js/ajax.js: {}