Fix errors in React Native children management

[sophiebits]

• Support using appendChild to move an existing child (Fiber does this, but we were assuming all children here were new; Yoga throws if you insert a child that already has a parent)
• Calculate beforeChildIndex after removing old child (previously, off by one when the new position is later than the old position)

[sophiebits]

Tests to come.

[bvaughn]

Nice catch, Ben.

Support using appendChild to move an existing child (Fiber does this, but we were assuming all children here were new; Yoga throws if you insert a child that already has a parent)

I would not have expected appendChild to be called for an already-appended child. I wonder if this will cause problems for other custom renderers?

A test or two would be nice! Otherwise LGTM

[sophiebits]

I can't figure out how to test the append thing easily but my test definitely covers the other one (adapted from a React ART test).

[gaearon]

Thanks for adding the test.