hook_flag_flag() is not working
To add some custom event on flag/unflag actions in Drupal 8 you need to use Event subscriber:

public static function getSubscribedEvents() {
    $events = [];
    $events[FlagEvents::ENTITY_FLAGGED][] = ['onFlag'];
    $events[FlagEvents::ENTITY_UNFLAGGED][] = ['onUnflag'];
    return $events;
}


And then in onFlag() and onUnflag() methods you can implement your custom logic.

I just wrote a blog post with some code examples: http://dev.studiopresent.com/blog/back-end/perform-actions-flag-unflag-drupal-8