Actually found it, it's not documented and not really foolproof:
$doctrineConfig = new Doctrine\ORM\Configuration();
However it will not work if the proxy autoloader is registered (which was necessary before, so if you forget to remove it, you'll get confusing errors that the proxy file can't be found), and you need to set a proxy dir else there's an exception:
exception 'Doctrine\Common\Proxy\Exception\InvalidArgumentException' with message 'You must configure a proxy directory. See docs for details'
The docs needs updating, both in code and on the website. If I find some time today I'll try to do it.