dillon1000/react

Don't mutate current tree before work is committed.

We should be able to abort an update without any side-effects to the
current tree. This fixes a few cases where that was broken.

The callback list should only ever be set on the workInProgress.
There's no reason to add it to the current tree because they're not
needed after they are called during the commit phase.

Also found a bug where the memoizedProps were set to null in the
case of an update, because the pendingProps were null. Fixed by
transfering the props from the instance, like we were already doing
with state.

Added a test to ensure that setState can be called inside a
callback.