Example of using defaultTargetPlatform to change the entire widget tree based on platform.

ios_android.dart

import 'package:flutter/cupertino.dart';
import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';


void main() {
	//runApp(IOSApp());
	//runApp(AndroidApp());
	runApp(defaultTargetPlatform == TargetPlatform.iOS ? IOSApp() : AndroidApp());
}

class IOSApp extends StatelessWidget {
	@override
	Widget build(BuildContext context) {
		return MaterialApp(
			theme: ThemeData(primaryColor: Colors.grey[100]),
			home: Material(
				child: CupertinoPageScaffold(
					backgroundColor: CupertinoColors.lightBackgroundGray,
					navigationBar: CupertinoNavigationBar(middle: Text('Example')),
					child: ListView.builder(itemBuilder: (BuildContext context, int index) {
						return GestureDetector(
							onTap: () => {},
							child: Container(
								margin: const EdgeInsets.symmetric(vertical: 8.0),
								padding: const EdgeInsets.symmetric(horizontal: 10.0),
								height: 50.0,
								decoration: const BoxDecoration(
									color: CupertinoColors.white,
									border: Border(
										bottom: BorderSide(color: Colors.black26, width: 0.0)
									),
								),
								child: Row(
									children: <Widget>[
										Expanded(
											child: Text('Item $index',
												style: const TextStyle(fontSize: 16.0, color: Colors.black),
											),
										),
										Icon(CupertinoIcons.right_chevron,
											color: CupertinoColors.inactiveGray,
										),
									],
								),
							),
						);
					}),
				),
			),
		);
	}
}

class AndroidApp extends StatelessWidget {
	@override
	Widget build(BuildContext context) {
		return MaterialApp(
			theme: ThemeData(primaryColor: Colors.grey[100]),
			home: Scaffold(
				backgroundColor: Colors.grey[200],
				appBar: AppBar(title: Text('Example')),
				body: ListView.builder(itemBuilder: (BuildContext context, int index) {
					return GestureDetector(
						onTap: () => {},
						child: Card(
							margin: const EdgeInsets.symmetric(vertical: 8.0, horizontal: 16.0),
							color: CupertinoColors.white,
							child: Container(
								padding: const EdgeInsets.symmetric(horizontal: 16.0),
								height: 56.0,
								child: Row(
									children: <Widget>[
										new Expanded(
											child: new Text('Item $index',
												style: new TextStyle(fontSize: 16.0, color: Colors.black),
											),
										),
										new Icon(Icons.chevron_right,
											color: Colors.grey,
										),
									],
								),
							),
						),
					);
				}),
			),
		);
	}
}