Need DRUPAL_ROOT in include of template.php

This should be fixed in the latest version.

The file check is now done when the Extension is created:

      if (!file_exists(dirname($pathname) . '/' . $filename)) {
        $filename = NULL;
      }
      $extension = new Extension($this->root, $type, $pathname, $filename);

And later in Extension::load()

  public function load() {
    if ($this->filename) {
      include_once $this->root . '/' . $this->getPath() . '/' . $this->filename;
      return TRUE;
    }
    return FALSE;
  }

... So we don't longer need it in d8