Analyzer soundness problem: Return void to Null allowed.

[eernstg]

Consider the following program:

void v = "Hello";

Null test() {
  return v;
}

void main() {
  int? i = test();
  if (i != null) {
    i.isEven;
  }
}

The analyzer (2.9.0-11.0.dev and 2eaac80) accepts this program with no errors, but it is not sound to allow an expression of type void to be returned when the return type is Null, because the returned object could have any type whatsoever.

A similar issue arises with the asynchronous form:

import 'dart:async';

void v = "Hello";

Future<Null> test() async {
  return v;
}

void main() async {
  int? i = await test();
  if (i != null) {
    i.isEven;
  }
}

[scheglov]

I believe that the issue was fixed by implementing changes requested in #41803.

I added a new test in https://dart-review.googlesource.com/c/sdk/+/149604.