Dart APIunself_componentNavigationContextexit exit property Null safety Widget? exit final Implementation final Widget? exit; unself_component unself_component NavigationContext exit property NavigationContext class Constructors NavigationContext Properties child exit hashCode key runtimeType Methods createElement debugDescribeChildren debugFillProperties noSuchMethod toDiagnosticsNode toString toStringDeep toStringShallow toStringShort updateShouldNotify Operators operator == Static methods of