Issue #2656900 by k4v: Use admin theme for file usage

Author: k4v

file_entity.services.yml

@@ -0,0 +1,5 @@
+services:
+  file_entity.route_subscriber:
+    class: Drupal\file_entity\Routing\RouteSubscriber
+    tags:
+      - { name: event_subscriber }

src/Routing/RouteSubscriber.php

@@ -0,0 +1,27 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\file_entity\Routing\RouteSubscriber.
+ */
+
+namespace Drupal\file_entity\Routing;
+
+use Drupal\Core\Routing\RouteSubscriberBase;
+use Symfony\Component\Routing\RouteCollection;
+
+/**
+ * Listens to the dynamic route events.
+ */
+class RouteSubscriber extends RouteSubscriberBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function alterRoutes(RouteCollection $collection) {
+    // The usage view should be shown in the admin theme.
+    if ($route = $collection->get('view.file_entity_files.usage')) {
+      $route->setOption('_admin_route', TRUE);
+    }
+  }
+}