Issue 3420768: base_url ignored in UnroutedUrlAssembler

Closes #3420768

core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php

@@ -131,7 +131,12 @@ protected function buildLocalUrl($uri, array $options = [], $collect_bubbleable_
     $current_base_path = $request->getBasePath() . '/';
 
     if ($options['absolute']) {
-      $current_base_url = $request->getSchemeAndHttpHost() . $current_base_path;
+      if (empty($options['base_url'])) {
+        $current_base_url = $request->getSchemeAndHttpHost() . $current_base_path;
+      }
+      else {
+        $current_base_url = rtrim($options['base_url'], '/') . $current_base_path;
+      }
       if (isset($options['https'])) {
         if (!empty($options['https'])) {
           $base = str_replace('http://', 'https://', $current_base_url);