Insert child into this render object's child list after the given child.
If after is null, then this inserts the child at the start of the list,
and the child becomes the new firstChild.
Source
void insert(ChildType child, { ChildType after }) {
assert(child != this);
assert(after != this);
assert(child != after);
assert(child != _firstChild);
assert(child != _lastChild);
adoptChild(child);
_insertIntoChildList(child, after: after);
}