Migrate platform_interaction to null safety (#80621)

dev/integration_tests/platform_interaction/lib/main.dart

@@ -16,7 +16,7 @@ void main() {
 }
 
 class TestApp extends StatefulWidget {
-  const TestApp({Key key}) : super(key: key);
+  const TestApp({Key? key}) : super(key: key);
 
   @override
   _TestAppState createState() => _TestAppState();
@@ -26,7 +26,7 @@ class _TestAppState extends State<TestApp> {
   static final List<TestStep> steps = <TestStep>[
     () => systemNavigatorPop(),
   ];
-  Future<TestStepResult> _result;
+  Future<TestStepResult>? _result;
   int _step = 0;
 
   void _executeNextStep() {

dev/integration_tests/platform_interaction/lib/src/system_navigation.dart

@@ -14,7 +14,7 @@ Future<TestStepResult> systemNavigatorPop() {
 
   final Completer<TestStepResult> completer = Completer<TestStepResult>();
 
-  channel.setMessageHandler((String message) async {
+  channel.setMessageHandler((String? message) async {
     completer.complete(
         const TestStepResult('System navigation pop', '', TestStatus.ok));
     return '';

dev/integration_tests/platform_interaction/lib/src/test_step.dart

@@ -23,12 +23,11 @@ class TestStepResult {
         return const TestStepResult('Executing', nothing, TestStatus.pending);
       case ConnectionState.done:
         if (snapshot.hasData) {
-          return snapshot.data;
+          return snapshot.data!;
         } else {
-          final TestStepResult result = snapshot.error as TestStepResult;
-          return result;
+          final Object? result = snapshot.error;
+          return result! as TestStepResult;
         }
-        break;
       default:
         throw 'Unsupported state ${snapshot.connectionState}';
     }

dev/integration_tests/platform_interaction/pubspec.yaml

@@ -2,7 +2,7 @@ name: platform_interaction
 description: Integration test for platform interactions.
 
 environment:
-  sdk: ">=2.0.0-dev.68.0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
 
 dependencies:
   flutter:

dev/integration_tests/platform_interaction/test_driver/main_test.dart

@@ -7,7 +7,7 @@ import 'package:test/test.dart' hide TypeMatcher, isInstanceOf;
 
 void main() {
   group('channel suite', () {
-    FlutterDriver driver;
+    late FlutterDriver driver;
 
     setUpAll(() async {
       driver = await FlutterDriver.connect(printCommunication: true);
@@ -28,7 +28,7 @@ void main() {
     });
 
     tearDownAll(() async {
-      driver?.close();
+      driver.close();
     });
   });
 }