align-items stretch behavior different with CSS spec

[fanhongwei]

when parent is align-items stretch and child set width or height，CSS Flex will use child own width or height, but flexbox-layout will use align-items stretch

[thagikura]

This is a specification bug. In CSS when a flex item has a size of width/height other than auto, that size is honored even if the parent's alignItems is set to stretch. But the stretch attribute always wins in FlexboxLayout.