diff --git a/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php
index c790a84..914809b 100644
--- a/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php
+++ b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php
@@ -31,4 +31,17 @@ function functionTemplateOverridden() {
     );
   }
 
+
+  /**
+   * Calls drupal_alter().
+   *
+   * This is for testing that the theme can have hook_*_alter() implementations
+   * that run during page callback execution, even before theme() is called for
+   * the first time.
+   */
+  function themeTestAlter() {
+    $data = 'foo';
+    drupal_alter('theme_test_alter', $data);
+    return "The altered data is $data.";
+  }
 }
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index 40a78f3..6ab8f04 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -54,8 +54,7 @@ function theme_test_menu() {
   );
   $items['theme-test/alter'] = array(
     'title' => 'Suggestion',
-    'page callback' => '_theme_test_alter',
-    'access callback' => TRUE,
+    'route_name' => 'theme_test_alter',
     'theme callback' => '_theme_custom_theme',
     'type' => MENU_CALLBACK,
   );
@@ -130,19 +129,6 @@ function _theme_custom_theme() {
 }
 
 /**
- * Page callback, calls drupal_alter().
- *
- * This is for testing that the theme can have hook_*_alter() implementations
- * that run during page callback execution, even before theme() is called for
- * the first time.
- */
-function _theme_test_alter() {
-  $data = 'foo';
-  drupal_alter('theme_test_alter', $data);
-  return "The altered data is $data.";
-}
-
-/**
  * Page callback, calls a theme hook suggestion.
  */
 function _theme_test_suggestion() {
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.routing.yml b/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
index 866171e..6fd74ae 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
+++ b/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
@@ -4,3 +4,10 @@ function_template_override:
     _content: '\Drupal\theme_test\ThemeTestController::functionTemplateOverridden'
   requirements:
     _permission: 'access content'
+
+theme_test_alter:
+  pattern: 'theme-test/alter'
+  defaults:
+    _content: '\Drupal\theme_test\ThemeTestController::themeTestAlter'
+  requirements:
+    _access: 'TRUE'