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

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

@@ -0,0 +1,17 @@
+{#
+/**
+ * @file
+ * Default theme implementation for a 'checkboxes' #type form element.
+ *
+ * Available variables
+ * - attributes: A list of HTML attributes for the wrapper element.
+ * - children: The rendered checkboxes.
+ *
+ * @see template_preprocess_checkboxes()
+ *
+ * @ingroup themeable
+ */
+ @todo: remove this file once https://www.drupal.org/node/1819284 is resolved.
+ This is identical to core/modules/system/templates/container.html.twig
+#}
+<div{{ attributes.addClass('form-checkboxes') }}>{{ children }}</div>