public function getParameter($key, $default_value = NULL) {
    if (isset($this->params[$key])) {
      return $this->params[$key];
    }

    $defaults = $this->sandbox()
      ->getPolicy()
      ->getParameterDefaults();

    if (isset($defaults[$key])) {
      $default_value = $defaults[$key];
    }

    // Ensure default values are recorded for use as tokens.
    $this->setParameter($key, $default_value);
    return $default_value;
  }