I don't see why it's an issue that Object.create is used, as long as attributes are assigned in a consistent order the optimization should still hold, since each assignment (or read) site would be monomorphic with respect to the current map (to use the Self terminology), even if the type isn't a useful indicator.

(If I'm horribly off let me know, I'm using my knowledge of PyPy to try to make some guesses about how precisely V8 applies these techniques).