measure
The extent the child takes under constraints.
A MeasurePolicy measures each child at most once per pass, and only in its measure block or in its placement block. A size it needs before choosing the constraints comes from the intrinsic functions. A LayoutModifierNode may measure its inner measurable again; each answer is then an independent Placeable.
Throws
if the policy already measured this child in the same pass, or measures it outside both blocks