1. override
void insertChildRenderObject(RenderObject child, Element slot)

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.

Source 

@override
void insertChildRenderObject(RenderObject child, Element slot) {
  final ContainerRenderObjectMixin<RenderObject, ContainerParentDataMixin<RenderObject>> renderObject = this.renderObject;
  renderObject.insert(child, after: slot?.renderObject);
  assert(renderObject == this.renderObject);
}