diff --git a/core/includes/menu.inc b/core/includes/menu.inc
index 251b81d..979c5fd 100644
--- a/core/includes/menu.inc
+++ b/core/includes/menu.inc
@@ -35,7 +35,7 @@ function template_preprocess_menu_local_task(&$variables) {
   $link_text = $link['title'];
 
   if (!empty($variables['element']['#active'])) {
-    $variables['attributes']['class'] = array('active');
+    $variables['active'] = TRUE;
 
     // Add text to indicate active tab for non-visual users.
     $active = String::format('<span class="visually-hidden">@label</span>', array('@label' => t('(active tab)')));
diff --git a/core/modules/file/file.module b/core/modules/file/file.module
index 0bc0584..f3eaca3 100644
--- a/core/modules/file/file.module
+++ b/core/modules/file/file.module
@@ -1193,7 +1193,6 @@ function template_preprocess_file_managed_file(&$variables) {
   if (!empty($element['#attributes']['class'])) {
     $variables['attributes']['class'] = (array) $element['#attributes']['class'];
   }
-  $variables['attributes']['class'][] = 'form-managed-file';
 }
 
 /**
diff --git a/core/themes/classy/templates/content-edit/file-managed-file.html.twig b/core/themes/classy/templates/content-edit/file-managed-file.html.twig
index 5e2f4e2..06bd21a 100644
--- a/core/themes/classy/templates/content-edit/file-managed-file.html.twig
+++ b/core/themes/classy/templates/content-edit/file-managed-file.html.twig
@@ -10,6 +10,6 @@
  * @see template_preprocess_file_managed_file()
  */
 #}
-<div{{ attributes }}>
+<div{{ attributes.addClass('form-managed-file') }}>
   {{ element }}
 </div>
diff --git a/core/themes/classy/templates/navigation/menu-local-task.html.twig b/core/themes/classy/templates/navigation/menu-local-task.html.twig
index d642199..5ab5a72 100644
--- a/core/themes/classy/templates/navigation/menu-local-task.html.twig
+++ b/core/themes/classy/templates/navigation/menu-local-task.html.twig
@@ -5,6 +5,7 @@
  *
  * Available variables:
  * - attributes: HTML attributes for the wrapper element.
+ * - active: Whether the task item is an active tab.
  * - link: A rendered link element.
  *
  * Note: This template renders the content for each task item in
@@ -13,4 +14,4 @@
  * @see template_preprocess_menu_local_task()
  */
 #}
-<li{{ attributes }}>{{ link }}</li>
+<li{{ attributes.addClass(active ? 'active') }}>{{ link }}</li>