Provide a method for checking whether a URL is for an unrouted, base-relative URL

I have found only one occurrence of strpos($this->uri, 'base://') !== 0) in core/Url.php. Is this issue still relevant?

Provide a method

We have that. It's isExternal() :)

In a few places

Per #2, it's down to one place, which is necessary, since isExternal() needs some implementation.

Is this issue still relevant?

I don't think so. Marking fixed since the other places have been fixed via some other issues after this was originally opened.

We have that. It's isExternal().

Correction: if you specifically want to know whether it's not routed and not external, you can call !$url->isRouted() && !$url->isExternal(). I don't think we need a single-method wrapper for that: those are two different checks so can remain distinct.

It seems like a good idea to remove the comment leading to this issue in \Drupal\Core\Url::setUnrouted(). Should we do it in this issue?

    // @todo Add a method for the check below in
    // https://www.drupal.org/node/2346859.
    if ($this->external = strpos($this->uri, 'base://') !== 0) {
      $this->options['external'] = TRUE;
    }

Just giving it a shot. I can't think of any other issue better suited to remove the comment.

Marking it as a quickfix, and a minor issue, which it is. :)