Insert the given entry into the overlay.
If above is non-null, the entry is inserted just above above.
Otherwise, the entry is inserted on top.
Source
void insert(OverlayEntry entry, { OverlayEntry above }) {
assert(entry._overlay == null);
assert(above == null || (above._overlay == this && _entries.contains(above)));
entry._overlay = this;
setState(() {
int index = above == null ? _entries.length : _entries.indexOf(above) + 1;
_entries.insert(index, entry);
});
}