Drupal 8: Dump name of all templates processed to filesystem

ThemeManager.php

# src/web/core/lib/Drupal/Core/Theme/ThemeManager.php, line 389
      
      static $c = 1;
      $filename = '/var/sys/RM26464.log';
      if ($c == 1) {
        $handle = fopen($filename, 'r+');
        ftruncate($handle, 0);
        rewind($handle);
        // echo fread($handle, filesize($filename));
        fclose($handle);
      }
      error_log("template $c: $template_file\n", 3, $filename);
      $c++;