Fix test in preparation of the Dart VM dropping support for language versions < 2.12.0 (#115176)

* Fix test in preparation of the Dart VM switching to being null safe by
default.

* Fix analyze error.

* Format.

* Remove print.

* Fix analyzer lint.

* Fix warnings.

packages/flutter/test/foundation/_compute_caller_unsound_null_safety.dart

@@ -2,19 +2,17 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart=2.9
 // Running in unsound null-safety mode is intended to test for potential miscasts
 // or invalid assertions.
 
 import 'package:flutter/src/foundation/_isolates_io.dart';
 import 'package:flutter/src/foundation/isolates.dart' as isolates;
 
-
-int returnInt(int arg) {
+int? returnInt(int? arg) {
   return arg;
 }
 
-Future<int> returnIntAsync(int arg) {
+Future<int?> returnIntAsync(int? arg) {
   return Future<int>.value(arg);
 }
 

packages/flutter/test/foundation/_compute_caller_unsound_null_safety_error.dart

@@ -2,21 +2,20 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart=2.9
 // Running in unsound null-safety mode is intended to test for potential miscasts
 // or invalid assertions.
 
 import 'package:flutter/src/foundation/_isolates_io.dart';
 
-int throwNull(int arg) {
-  throw null;
+int throwNull(dynamic arg) {
+  throw arg as Object;
 }
 
 void main() async {
   try {
     await compute(throwNull, null);
   } catch (e) {
-    if (e is! NullThrownError) {
+    if (e is! TypeError && e is! NullThrownError) {
       throw Exception('compute returned bad result');
     }
   }

packages/flutter/test/foundation/isolates_test.dart

@@ -80,19 +80,25 @@ Future<int> test5CallCompute(int value) {
   return compute(test5, value);
 }
 
-Future<void> expectFileSuccessfullyCompletes(String filename) async {
+Future<void> expectFileSuccessfullyCompletes(String filename,
+    [bool unsound = false]) async {
   // Run a Dart script that calls compute().
   // The Dart process will terminate only if the script exits cleanly with
   // all isolate ports closed.
   const FileSystem fs = LocalFileSystem();
   const Platform platform = LocalPlatform();
   final String flutterRoot = platform.environment['FLUTTER_ROOT']!;
-  final String dartPath = fs.path.join(flutterRoot, 'bin', 'cache', 'dart-sdk', 'bin', 'dart');
+  final String dartPath =
+      fs.path.join(flutterRoot, 'bin', 'cache', 'dart-sdk', 'bin', 'dart');
   final String packageRoot = fs.path.dirname(fs.path.fromUri(platform.script));
-  final String scriptPath = fs.path.join(packageRoot, 'test', 'foundation', filename);
+  final String scriptPath =
+      fs.path.join(packageRoot, 'test', 'foundation', filename);
+  final String nullSafetyArg =
+      unsound ? '--no-sound-null-safety' : '--sound-null-safety';
 
   // Enable asserts to also catch potentially invalid assertions.
-  final ProcessResult result = await Process.run(dartPath, <String>['run', '--enable-asserts', scriptPath]);
+  final ProcessResult result = await Process.run(
+      dartPath, <String>[nullSafetyArg, 'run', '--enable-asserts', scriptPath]);
   expect(result.exitCode, 0);
 }
 
@@ -194,7 +200,8 @@ void main() {
     expect(await computeInstanceMethod(10), 100);
     expect(computeInvalidInstanceMethod(10), throwsArgumentError);
 
-    expect(await compute(testDebugName, null, debugLabel: 'debug_name'), 'debug_name');
+    expect(await compute(testDebugName, null, debugLabel: 'debug_name'),
+        'debug_name');
     expect(await compute(testReturnNull, null), null);
   }, skip: kIsWeb); // [intended] isn't supported on the web.
 
@@ -203,22 +210,26 @@ void main() {
       await expectFileSuccessfullyCompletes('_compute_caller.dart');
     });
     test('with invalid message', () async {
-      await expectFileSuccessfullyCompletes('_compute_caller_invalid_message.dart');
+      await expectFileSuccessfullyCompletes(
+          '_compute_caller_invalid_message.dart');
     });
     test('with valid error', () async {
       await expectFileSuccessfullyCompletes('_compute_caller.dart');
     });
     test('with invalid error', () async {
-      await expectFileSuccessfullyCompletes('_compute_caller_invalid_message.dart');
+      await expectFileSuccessfullyCompletes(
+          '_compute_caller_invalid_message.dart');
     });
   }, skip: kIsWeb); // [intended] isn't supported on the web.
 
   group('compute() works with unsound null safety caller', () {
     test('returning', () async {
-      await expectFileSuccessfullyCompletes('_compute_caller_unsound_null_safety.dart');
+      await expectFileSuccessfullyCompletes(
+          '_compute_caller_unsound_null_safety.dart', true);
     });
     test('erroring', () async {
-      await expectFileSuccessfullyCompletes('_compute_caller_unsound_null_safety_error.dart');
+      await expectFileSuccessfullyCompletes(
+          '_compute_caller_unsound_null_safety_error.dart', true);
     });
   }, skip: kIsWeb); // [intended] isn't supported on the web.
 }