Flow analysis doesn't recognize that null check patterns always match a non-nullable type

[stereotype441]

Flow analysis currently (as of cb4c9c7) fails to recognize that if the matched value type is non-nullable, then a null check pattern (e.g. _?) or the equivalent relational pattern (!= null), will always match.

As a result, flow analysis can't recognize that a switch statement like this one is trivially exhaustive:

bool f(int i) { // (1)
  switch (i) {
    case var _?:
      return i.isEven;
  }
}

main() {
  print(f(0));
  print(f(1));
}

And so an error is issued at (1), saying "The body might complete normally, causing 'null' to be returned, but the return type, 'bool', is a potentially non-nullable type."

If flow analysis were slightly smarter, it would see that no error needs to be issued at (1), because var _? always matches the non-nullable type int, so there would be no error.