--- /dev/null
+<?php
+
+namespace Drupal\system\EventSubscriber;
+
+use Drupal\Core\Routing\RouteSubscriberBase;
+use Drupal\Core\Routing\RoutingEvents;
+use Symfony\Component\Routing\RouteCollection;
+
+/**
+ * Adds the _admin_route option to each admin route.
+ */
+class AdminRouteSubscriber extends RouteSubscriberBase {
+
+ /**
+ * {@inheritdoc}
+ */
+ protected function alterRoutes(RouteCollection $collection) {
+ foreach ($collection->all() as $route) {
+ if (strpos($route->getPath(), '/admin') === 0 && !$route->hasOption('_admin_route')) {
+ $route->setOption('_admin_route', TRUE);
+ }
+ }
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public static function getSubscribedEvents() {
+ $events = parent::getSubscribedEvents();
+
+ // Use a lower priority than \Drupal\field_ui\Routing\RouteSubscriber or
+ // \Drupal\views\EventSubscriber\RouteSubscriber to ensure we add the option
+ // to their routes.
+ $events[RoutingEvents::ALTER] = ['onAlterRoutes', -200];
+
+ return $events;
+ }
+
+}