Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

core/themes/bartik/templates/region--header.html.twig

@@ -0,0 +1,16 @@
+{% extends "region.html.twig" %}
+{#
+/**
+ * @file
+ * Bartik's theme implementation to display a header region.
+ *
+ * Available variables:
+ * - content: The content for this region, typically blocks.
+ * - attributes: HTML attributes for the region div.
+ * - region: The name of the region variable as defined in the theme's
+ *   .info.yml file.
+ *
+ * @see template_preprocess_region()
+ */
+#}
+{% set attributes = attributes.addClass('clearfix') %}