FadeIn/FadeOut page transition in flutter

Question

I am stack at easy problem whitch i am unable to solve. I want to create easy fadein/out page transition in flutter web, but for some reason the transition only occure as FadeIn. Previous page does not fade out. Here is a code:

PageRoute _getPageRoute(Widget child) {
  return _FadeRoute(child: child);
}

class _FadeRoute extends PageRouteBuilder {
  final Widget child;
  _FadeRoute({this.child})
      : super(
          pageBuilder: (
            BuildContext context,
            Animation<double> animation,
            Animation<double> secondaryAnimation,
          ) =>
              child,
          transitionsBuilder: (
            BuildContext context,
            Animation<double> animation,
            Animation<double> secondaryAnimation,
            Widget child,
          ) =>
              FadeTransition(
            opacity: animation,
            child: child,
          ),
        );
}

I have searched for hours but coudn't resolved the issue. If anyone of you have any idea, please, dont hesitate to answer! Thanks in advance!