Update to not use _mountImage

[sophiebits]

See facebook/react#852.

Unfortunately I'm not at all set up to run and test this.

[sebmarkbage]

There shouldn't be a need for another lookup map. Those are not free. We should be able to just store a _mountImage on the child like we used to here. E.g. assign it in createChild.

Note that this is expensive paths for ART because move gets called for every child every time, not just for actual moves.

[sophiebits]

Better?

[sebmarkbage]

delete tends to trigger a deopt in heuristics that assume that these are used as maps instead of objects. It's better to reset it to null so that it retains it's hidden class shape.

[sophiebits]

done.

[sebmarkbage]

hm... This fails the unit test. Not really sure how to get this test running in the open-source environment. It can probably be dropped into the react repo. :/

[sophiebits]

I tried doing that and ran into trouble requiring art. I can look more later.

[sophiebits]

Just updated -- I forgot to store _mountImage on initial mount. Now passes the tests (at least with my runner: #5).