removePopStateListener method Null safety
override
Unregisters the given listener (added by addPopStateListener) from the
popstate
event.
See: https://developer.mozilla.org/en-US/docs/Web/API/WindowEventHandlers/onpopstate
Implementation
@override
void removePopStateListener(html.EventListener fn) {
html.window.removeEventListener('popstate', fn);
}