Summary:
We currently have a bug in `UIView+CSSLayout.m` that you can repro in a scenario like this:
```
UIView *container = [[UIView alloc] initWithFrame:CGRectZero];
[container css_setUsesFlexbox:YES];
[container css_setFlexDirection:CSSFlexDirectionRow];
UIView *subview1 = [[UIView alloc] initWithFrame:CGRectZero];
[subview1 css_setUsesFlexbox:YES];
[subview1 css_setIncludeInLayout:NO];
[container addSubview:subview1];
UIView *subview2 = [[UIView alloc] initWithFrame:CGRectZero];
[subview2 css_setUsesFlexbox:YES];
[subview2 css_setIncludeInLayout:NO];
[container addSubview:subview2];
UIView *subview3 = [[UIView alloc] initWithFrame:CGRectZero];
[subview3 css_setUsesFlexbox:YES];
[subview3 css_setIncludeInLayout:YES];
[container addSubview:subview3];
[container css_applyLayout];
```
`subview3` (which is the only view whose is going to be included in layout calculation) is inserted at child index 2, instead of 0. This eventually can cause crash in CSSLayout.c because it will attempt access a child in the list which is null.
Reviewed By: emilsjolander
Differential Revision: D4215659
fbshipit-source-id: a615f50e51f85b15d3bdb437e55958865898b183
49a21e657b