Added sample code for multiple flutters on iOS (#670)

add_to_app/multiple_flutters/multiple_flutters_module/lib/main.dart

@@ -0,0 +1,98 @@
+// Copyright 2021 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+
+void main() => runApp(MyApp(Colors.blue));
+
+@pragma('vm:entry-point')
+void topMain() => runApp(MyApp(Colors.green));
+
+@pragma('vm:entry-point')
+void bottomMain() => runApp(MyApp(Colors.purple));
+
+class MyApp extends StatelessWidget {
+  MyApp(this.color);
+
+  final Color color;
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Flutter Demo',
+      theme: ThemeData(
+        primarySwatch: this.color,
+      ),
+      home: MyHomePage(title: 'Flutter Demo Home Page'),
+    );
+  }
+}
+
+class MyHomePage extends StatefulWidget {
+  MyHomePage({Key key, this.title}) : super(key: key);
+  final String title;
+
+  @override
+  _MyHomePageState createState() => _MyHomePageState();
+}
+
+class _MyHomePageState extends State<MyHomePage> {
+  int _counter = 0;
+  MethodChannel _channel;
+
+  @override
+  void initState() {
+    super.initState();
+    _channel = MethodChannel('multiple-flutters');
+    _channel.setMethodCallHandler((MethodCall call) async {
+      if (call.method == "setCount") {
+        // A notification that the host platform's data model has been updated.
+        setState(() {
+          _counter = call.arguments as int;
+        });
+      } else {
+        throw Exception('not implemented ${call.method}');
+      }
+    });
+  }
+
+  void _incrementCounter() {
+    // Mutations to the data model are forwarded to the host platform.
+    _channel.invokeMethod("incrementCount", _counter);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text(widget.title),
+      ),
+      body: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: <Widget>[
+            Text(
+              'You have pushed the button this many times:',
+            ),
+            Text(
+              '$_counter',
+              style: Theme.of(context).textTheme.headline4,
+            ),
+            TextButton(
+              onPressed: _incrementCounter,
+              child: Text('Add'),
+            ),
+            TextButton(
+              onPressed: () {
+                _channel.invokeMethod("next", _counter);
+              },
+              child: Text('Next'),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}