FakeDevicePixelRatio

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(
    FakeDevicePixelRatio(
      fakeDevicePixelRatio: 1.5,
      child: MyApp(),
    ),
  );
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('foo')),
        body: Center(
          child: Text('foo'),
        ),
      ),
    );
  }
}

class FakeDevicePixelRatio extends StatelessWidget {
  final num fakeDevicePixelRatio;
  final Widget child;

  FakeDevicePixelRatio({this.fakeDevicePixelRatio, this.child})
      : assert(fakeDevicePixelRatio != null);

  @override
  Widget build(BuildContext context) {
    final ratio =
        fakeDevicePixelRatio / WidgetsBinding.instance.window.devicePixelRatio;

    return FractionallySizedBox(
      widthFactor: 1 / ratio,
      heightFactor: 1 / ratio,
      child: Transform.scale(
        scale: ratio,
        child: child,
      ),
    );
  }
}