Fix `compute` in unsound null safety debug mode (#100544)

packages/flutter/lib/src/foundation/_isolates_io.dart

@@ -65,7 +65,6 @@ Future<R> compute<Q, R>(isolates.ComputeCallback<Q, R> callback, Q message, { St
   switch (type) {
     // success; see _buildSuccessResponse
     case 1:
-      assert(response[0] is R);
       return response[0] as R;
 
     // native error; see Isolate.addErrorListener
@@ -78,10 +77,7 @@ Future<R> compute<Q, R>(isolates.ComputeCallback<Q, R> callback, Q message, { St
     // caught error; see _buildErrorResponse
     case 3:
     default:
-      assert(type == 3);
-      assert(response[0] is Object);
-      assert(response[1] is StackTrace);
-      assert(response[2] == null);
+      assert(type == 3 && response[2] == null);
 
       await Future<Never>.error(
         response[0] as Object,

packages/flutter/test/foundation/_compute_caller_unsound_null_safety.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// 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) {
+  return arg;
+}
+
+Future<int> returnIntAsync(int arg) {
+  return Future<int>.value(arg);
+}
+
+Future<void> testCompute<T>(isolates.ComputeCallback<T, T> callback, T input) async {
+  if (input != await compute(callback, input)) {
+    throw Exception('compute returned bad result');
+  }
+}
+
+void main() async {
+  await testCompute(returnInt, 10);
+  await testCompute(returnInt, null);
+  await testCompute(returnIntAsync, 10);
+  await testCompute(returnIntAsync, null);
+}

packages/flutter/test/foundation/_compute_caller_unsound_null_safety_error.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// 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;
+}
+
+void main() async {
+  try {
+    await compute(throwNull, null);
+  } catch (e) {
+    if (e is! NullThrownError) {
+      throw Exception('compute returned bad result');
+    }
+  }
+}

packages/flutter/test/foundation/isolates_test.dart

@@ -80,7 +80,7 @@ Future<int> test5CallCompute(int value) {
   return compute(test5, value);
 }
 
-Future<void> expectFileClosesAllPorts(String filename) async {
+Future<void> expectFileSuccessfullyCompletes(String filename) async {
   // Run a Dart script that calls compute().
   // The Dart process will terminate only if the script exits cleanly with
   // all isolate ports closed.
@@ -90,7 +90,9 @@ Future<void> expectFileClosesAllPorts(String filename) async {
   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 ProcessResult result = await Process.run(dartPath, <String>[scriptPath]);
+
+  // Enable asserts to also catch potentially invalid assertions.
+  final ProcessResult result = await Process.run(dartPath, <String>['run', '--enable-asserts', scriptPath]);
   expect(result.exitCode, 0);
 }
 
@@ -159,6 +161,10 @@ String? testDebugName(_) {
   return Isolate.current.debugName;
 }
 
+int? testReturnNull(_) {
+  return null;
+}
+
 void main() {
   test('compute()', () async {
     expect(await compute(test1, 0), 1);
@@ -189,20 +195,30 @@ void main() {
     expect(computeInvalidInstanceMethod(10), throwsArgumentError);
 
     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.
 
-  group('compute closes all ports', () {
+  group('compute() closes all ports', () {
     test('with valid message', () async {
-      await expectFileClosesAllPorts('_compute_caller.dart');
+      await expectFileSuccessfullyCompletes('_compute_caller.dart');
     });
     test('with invalid message', () async {
-      await expectFileClosesAllPorts('_compute_caller_invalid_message.dart');
+      await expectFileSuccessfullyCompletes('_compute_caller_invalid_message.dart');
     });
     test('with valid error', () async {
-      await expectFileClosesAllPorts('_compute_caller.dart');
+      await expectFileSuccessfullyCompletes('_compute_caller.dart');
     });
     test('with invalid error', () async {
-      await expectFileClosesAllPorts('_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');
+    });
+    test('erroring', () async {
+      await expectFileSuccessfullyCompletes('_compute_caller_unsound_null_safety_error.dart');
     });
   }, skip: kIsWeb); // [intended] isn't supported on the web.
 }