Migrate multiple_flutters to null safety (#80630)

dev/benchmarks/multiple_flutters/module/lib/main.dart

@@ -13,7 +13,7 @@ void topMain() => runApp(const MyApp(Colors.green));
 void bottomMain() => runApp(const MyApp(Colors.purple));
 
 class MyApp extends StatelessWidget {
-  const MyApp(this.color, {Key key}) : super(key: key);
+  const MyApp(this.color, {Key? key}) : super(key: key);
 
   final Color color;
 
@@ -30,8 +30,8 @@ class MyApp extends StatelessWidget {
 }
 
 class MyHomePage extends StatefulWidget {
-  const MyHomePage({Key key, this.title}) : super(key: key);
-  final String title;
+  const MyHomePage({Key? key, this.title}) : super(key: key);
+  final String? title;
 
   @override
   _MyHomePageState createState() => _MyHomePageState();
@@ -42,7 +42,7 @@ class _MyHomePageState extends State<MyHomePage> {
   Widget build(BuildContext context) {
     return Scaffold(
       appBar: AppBar(
-        title: Text(widget.title),
+        title: Text(widget.title ?? ''),
       ),
       body: Center(
         child: Column(

dev/benchmarks/multiple_flutters/module/pubspec.yaml

@@ -4,7 +4,7 @@ description: A module that is embedded in the multiple_flutters benchmark test.
 version: 1.0.0+1
 
 environment:
-  sdk: '>=2.8.1 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   flutter: