Navigator({Key key, String initialRoute, @required RouteFactory onGenerateRoute, RouteFactory onUnknownRoute, NavigatorObserver observer })

Creates a widget that maintains a stack-based history of child widgets.

The onGenerateRoute argument must not be null.

Source 

Navigator({
  Key key,
  this.initialRoute,
  @required this.onGenerateRoute,
  this.onUnknownRoute,
  this.observer
}) : super(key: key) {
  assert(onGenerateRoute != null);
}