Migrate the ios_app_with_extensions to null safety (#80485)

dev/integration_tests/ios_app_with_extensions/lib/main.dart

@@ -7,7 +7,7 @@ import 'package:flutter/material.dart';
 void main() => runApp(const MyApp());
 
 class MyApp extends StatelessWidget {
-  const MyApp({Key key}) : super(key: key);
+  const MyApp({Key? key}) : super(key: key);
 
   // This widget is the root of your application.
   @override
@@ -32,7 +32,7 @@ class MyApp extends StatelessWidget {
 }
 
 class MyHomePage extends StatefulWidget {
-  const MyHomePage({Key key, this.title}) : super(key: key);
+  const MyHomePage({Key? key, this.title}) : super(key: key);
 
   // This widget is the home page of your application. It is stateful, meaning
   // that it has a State object (defined below) that contains fields that affect
@@ -43,7 +43,7 @@ class MyHomePage extends StatefulWidget {
   // used by the build method of the State. Fields in a Widget subclass are
   // always marked "final".
 
-  final String title;
+  final String? title;
 
   @override
   _MyHomePageState createState() => _MyHomePageState();
@@ -75,7 +75,7 @@ class _MyHomePageState extends State<MyHomePage> {
       appBar: AppBar(
         // Here we take the value from the MyHomePage object that was created by
         // the App.build method, and use it to set our appbar title.
-        title: Text(widget.title),
+        title: Text(widget.title ?? ''),
       ),
       body: Center(
         // Center is a layout widget. It takes a single child and positions it

dev/integration_tests/ios_app_with_extensions/pubspec.yaml

@@ -13,7 +13,7 @@ name: ios_app_with_extensions
 version: 1.0.0+1
 
 environment:
-  sdk: ">=2.1.0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
 
 dependencies:
   flutter: