buildExternalUrl example

  public function assemble($uri, array $options = [], $collect_bubbleable_metadata = FALSE) {
    // Note that UrlHelper::isExternal will return FALSE if the $uri has a     // disallowed protocol. This is later made safe since we always add at     // least a leading slash.     if (parse_url($uri, PHP_URL_SCHEME) === 'base') {
      return $this->buildLocalUrl($uri, $options, $collect_bubbleable_metadata);
    }
    elseif (UrlHelper::isExternal($uri)) {
      // UrlHelper::isExternal() only returns true for safe protocols.       return $this->buildExternalUrl($uri, $options, $collect_bubbleable_metadata);
    }
    throw new \InvalidArgumentException("The URI '$uri' is invalid. You must use a valid URI scheme. Use base: for a path, e.g., to a Drupal file that needs the base path. Do not use this for internal paths controlled by Drupal.");
  }

  /** * {@inheritdoc} */
  protected function buildExternalUrl($uri, array $options = [], $collect_bubbleable_metadata = FALSE) {
    $this->addOptionDefaults($options);
    // Split off the query & fragment.     $parsed = UrlHelper::parse($uri);