RenderUnconstrainedBox constructor
Create a render object that sizes itself to the child but does not pass the constraints down to that child.
The alignment must not be null.
Implementation
RenderUnconstrainedBox({
@required AlignmentGeometry alignment,
@required TextDirection textDirection,
Axis constrainedAxis,
RenderBox child,
}) : assert(alignment != null),
_constrainedAxis = constrainedAxis,
super.mixin(alignment, textDirection, child);