Move the given child to the given slot.
The given child is guaranteed to have renderObject as its parent.
The semantics of
slot are determined by this element. For example, if
this element has a single child, the slot should always be null. If this
element has a list of children, the previous sibling is a convenient value
for the slot.
@protected void moveChildRenderObject(covariant RenderObject child, covariant dynamic slot);