Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/system/templates/container.html.twig

@@ -0,0 +1,26 @@
+{#
+/**
+ * @file
+ * Default theme implementation of a container used to wrap child elements.
+ *
+ * Used for grouped form items. Can also be used as a #theme_wrapper for any
+ * renderable element, to surround it with a <div> and HTML attributes.
+ *
+ * Available variables:
+ * - attributes: HTML attributes for the containing element.
+ * - children: The rendered child elements of the container.
+ * - has_parent: A flag to indicate that the container has one or more parent
+     containers.
+ *
+ * @see template_preprocess_container()
+ *
+ * @ingroup themeable
+ */
+#}
+{%
+  set classes = [
+    has_parent ? 'js-form-wrapper',
+    has_parent ? 'form-wrapper',
+  ]
+%}
+<div{{ attributes.addClass(classes) }}>{{ children }}</div>