homepage.dart

@override
  Widget build(BuildContext context) {
    return ResponsiveWidget(
      largeScreen: Scaffold(
        appBar: ResponsiveWidget.isSmallScreen(context) 
        ?AppBar(
          leading: ClipRRect(
            child: Container(
                height: 20,
                width: 20,
                child: Text("small screen"),
          ),
          actions: <Widget>[],
          elevation: 0.0,
        ):
          AppBar(
          leading: ClipRRect(
            child: Container(
                height: 20,
                width: 20,
                child: Text("large screen"),
          ),
          actions: <Widget>[],
          elevation: 0.0,
        )
        body: ResponsiveWidget.isSmallScreen(context)
            ? smallScreen()
            : largeScreen(),
      ),
);

responsivewidget.dart

import 'package:flutter_web/material.dart';

class ResponsiveWidget extends StatelessWidget {
  final Widget largeScreen;
  final Widget mediumScreen;
  final Widget smallScreen;

  const ResponsiveWidget(
      {Key key,
      @required this.largeScreen,
      this.mediumScreen,
      this.smallScreen})
      : super(key: key);

  static bool isSmallScreen(BuildContext context) {
    return MediaQuery.of(context).size.width < 800;
  }

  static bool isLargeScreen(BuildContext context) {
    return MediaQuery.of(context).size.width > 800;
  }

  static bool isMediumScreen(BuildContext context) {
    return MediaQuery.of(context).size.width >= 800 &&
            MediaQuery.of(context).size.width <= 1200;
  }

  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(
      builder: (context, constraints) {
        if (constraints.maxWidth > 1200) {
          return largeScreen;
        } else if (constraints.maxWidth <= 1200 && constraints.maxWidth >= 800) {
          return mediumScreen ?? largeScreen;
        } else {
          return smallScreen ?? largeScreen;
        }
      },
    );
  }
}