dart-lang#2956. Add more type void tests

Language/Functions/Type_of_a_Function/return_type_t04.dart

@@ -0,0 +1,46 @@
+// Copyright (c) 2024, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// @assertion If a function declaration does not declare a return type
+/// explicitly, its return type is dynamic, unless it is a constructor, in which
+/// case it is not considered to have a return type, or it is a setter or
+/// operator []=, in which case its return type is void
+///
+/// @description Checks that if a setter or operator `[]=` declaration does not
+/// declare a return type then the return type is `void` and it's a compile-time
+/// error to return a value from this declaration.
+/// @author sgrekhov22@gmail.com
+
+set globalSetter(int _) {
+  return 0;
+//       ^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+class C {
+  operator []=(int index, Object? value) {
+    return 0;
+//         ^
+// [analyzer] unspecified
+// [cfe] unspecified
+  }
+  static set staticSetter(int _) {
+    return 0;
+//         ^
+// [analyzer] unspecified
+// [cfe] unspecified
+  }
+  set instanceSetter(int _) {
+    return 0;
+//         ^
+// [analyzer] unspecified
+// [cfe] unspecified
+  }
+}
+
+main() {
+  globalSetter = 1;
+  print(C);
+}

Language/Functions/function_body_short_syntax_A03_t04.dart

@@ -0,0 +1,34 @@
+// Copyright (c) 2024, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// @assertion A function body is either:
+/// ...
+/// - of the form `=> e` or the form `async => e`, which both return the value
+///   of the expression `e` as if by a `return e`. ...  Let `T` be the declared
+///   return type of the function that has this body. It is a compile-time error
+///   if one of the following conditions hold:
+///   – The function is synchronous, `T` is not `void`, and it would have been a
+///     compile-time error to declare the function with the body `{ return e; }`
+///     rather than `=> e`.
+///
+/// @description Checks that it is not an error to declare a synchronous
+/// function with a short syntax and the return type `FutureOr<void>`.
+/// @author sgrekhov22@qmail.com
+
+import 'dart:async';
+
+FutureOr<void> f1() => 1;
+FutureOr<void> f2() => null;
+FutureOr<void> f3<T>(T t) => t;
+
+FutureOr<void> f4() {
+  return 1;
+}
+
+main() {
+  print(f1);
+  print(f2);
+  print(f3);
+  print(f4);
+}