diff --git a/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php b/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
index c5a10b8..196a059 100644
--- a/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
+++ b/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
@@ -8,6 +8,8 @@
 namespace Drupal\system_test\Controller;
 
 use Drupal\Core\Controller\ControllerBase;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\Response;
 
 /**
  * Controller routines for system_test routes.
@@ -70,10 +72,15 @@ public function authorizeInit($page_title) {
   }
 
   /**
-   * @todo Remove system_test_set_header().
+   * Sets a header.
    */
-  public function setHeader() {
-    return system_test_set_header();
+  public function setHeader(Request $request) {
+    $query = $request->query->all();
+    $response = new Response();
+    $response->headers->set($query['name'], $query['value']);
+    $response->setContent($this->t('The following header was set: %name: %value', array('%name' => $query['name'], '%value' => $query['value'])));
+
+    return $response;
   }
 
   /**
diff --git a/core/modules/system/tests/modules/system_test/system_test.module b/core/modules/system/tests/modules/system_test/system_test.module
index 69d9a16..40da5d2 100644
--- a/core/modules/system/tests/modules/system_test/system_test.module
+++ b/core/modules/system/tests/modules/system_test/system_test.module
@@ -2,19 +2,6 @@
 
 use Drupal\Core\Extension\Extension;
 use Symfony\Component\HttpFoundation\RedirectResponse;
-use Symfony\Component\HttpFoundation\Response;
-
-/**
- * Sets a header.
- *
- * @deprecated \Drupal\system_test\Controller\SystemTestController::setHeader()
- */
-function system_test_set_header() {
-  $query = \Drupal::request()->query->all();
-  $response = new Response();
-  $response->headers->set($query['name'], $query['value']);
-  return t('The following header was set: %name: %value', array('%name' => $query['name'], '%value' => $query['value']));
-}
 
 /**
  * Implements hook_modules_installed().