Update abstract_method_smoke_test to null safety (#80555)

dev/integration_tests/abstract_method_smoke_test/lib/main.dart

@@ -14,7 +14,7 @@ Future<void> main() async {
 }
 
 class MyApp extends StatelessWidget {
-  const MyApp({Key key}) : super(key: key);
+  const MyApp({Key? key}) : super(key: key);
 
   @override
   Widget build(BuildContext context) {
@@ -29,7 +29,7 @@ class MyApp extends StatelessWidget {
 }
 
 class HomePage extends StatefulWidget {
-  const HomePage({Key key}) : super(key: key);
+  const HomePage({Key? key}) : super(key: key);
 
   @override
   _HomePage createState() => _HomePage();
@@ -42,7 +42,7 @@ class _HomePage extends State<HomePage> {
 
     // Trigger the second route.
     // https://github.com/flutter/flutter/issues/40126
-    WidgetsBinding.instance.addPostFrameCallback((_) async {
+    WidgetsBinding.instance?.addPostFrameCallback((_) async {
       Navigator.of(context).push(
           MaterialPageRoute<void>(builder: (_) => const SecondPage()));
     });
@@ -55,7 +55,7 @@ class _HomePage extends State<HomePage> {
 }
 
 class SecondPage extends StatelessWidget {
-  const SecondPage({Key key}) : super(key: key);
+  const SecondPage({Key? key}) : super(key: key);
 
   @override
   Widget build(BuildContext context) {

dev/integration_tests/abstract_method_smoke_test/pubspec.yaml

@@ -4,7 +4,7 @@ description: A new Flutter project.
 version: 1.0.0+1
 
 environment:
-  sdk: ">=2.1.0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
 
 dependencies:
   flutter: