Drupal 8 - Set active class name for active link.

set_active.php

function theme_preprocess_menu(&$variables, $hook) {
  if ($hook == 'menu') {
    $current_path = \Drupal::request()->getRequestUri();
    $items = $variables['items'];

    foreach ($items as $key => $item) {
      // Set active to dom element if path of menu item matches current path
      if ($item['url']->toString() == $current_path) {
        // Add active link.
        $variables['items'][$key]['attributes']['class'][] = 'active';
      } else {
        // Set active to dom element if path of menu item matches first part of current path
        $url_fragments = explode('/', $current_path);
        if (count($url_fragments) > 1 AND '/' . $url_fragments[1] == $item['url']->toString()) {
          $variables['items'][$key]['attributes']['class'][] = 'active';
        }
      }
    }
  }
}

// $variables['items'][$key]['attributes']['class'][] = 'active';
// set to
// $variables['items'][$key]['attributes']->addClass('active');