GeneratedButton example

    if ($url->isExternal()) {
      $generated_link = new GeneratedLink();
      $attributes['href'] = $url->toString(FALSE);
      return $this->doGenerate($generated_link, $attributes, $variables);
    }
    if ($url->isRouted() && $url->getRouteName() === '<nolink>') {
      $generated_link = new GeneratedNoLink();
      unset($attributes['href'], $attributes['hreflang']);
      return $this->doGenerate($generated_link, $attributes, $variables);
    }
    if ($url->isRouted() && $url->getRouteName() === '<button>') {
      $generated_link = new GeneratedButton();
      $attributes['type'] = 'button';
      unset($attributes['href'], $attributes['hreflang']);
      return $this->doGenerate($generated_link, $attributes, $variables);
    }
    $generated_url = $url->toString(TRUE);
    $generated_link = GeneratedLink::createFromObject($generated_url);
    // The result of the URL generator is a plain-text URL to use as the href     // attribute, and it is escaped by \Drupal\Core\Template\Attribute.     $attributes['href'] = $generated_url->getGeneratedUrl();
    return $this->doGenerate($generated_link, $attributes, $variables);
  }