Does flooring need to be done before built-in wardrobes?
I would recommend that any new walls, including those for built-in closets/wardrobes be done before the final finish flooring goes down.
You would want to build on the subfloor, nailing the sole plate of the wall down to the subfloor (preferably into joists below, if at all possible), and not having to worry about trying to cut through finish flooring in 10 or 20 years when you (or someone else) want to redo the floors again.