insertChildRenderObject method
- @override
override
    Insert the given child into renderObject at the given slot.
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.
Implementation
@override
void insertChildRenderObject(RenderObject child, int slot) {
  final RenderListWheelViewport renderObject = this.renderObject;
  assert(renderObject.debugValidateChild(child));
  renderObject.insert(child, after: _childElements[slot - 1]?.renderObject);
  assert(renderObject == this.renderObject);
}