Errors instead of static warnings in analyzer for Setter declarations issues

[sgrekhov]

According to the Dart Language Specification (Version 2.8.0-dev, April 22, 2020) 10.3 Setters

It is a static warning if a setter declares a return type other than void. It is
a static warning if a class has a setter named v= with argument type T and a
getter named v with return type S, and S may not be assigned to T.

In fact there are errors in analyzer in both of these cases and no issues in VM

class C {
  int set foo(var x) => 42; // In Analyzer 'error - The return type of the setter must be 'void' or absent.'  and no issues in VM
}

main() {
  new C().foo = 1;
}

class C<T extends int, S extends String> {
  set foo(T t) {
    _foo = t;
  }

  S get foo { return _foo; } // In Analyzer error - The return type of getter 'foo' is 'S' which isn't assignable to the type 'T' of its setter 'foo' and no issues in VM

  var _foo;
}

main() {
  new C<int, String>().foo = 42;
}

According to the specification there must be a static warnings in both of these cases not an errors
dartanalyzer version 2.9.0-13.0.dev

[eernstg]

Cf. dart-lang/language#1012.