alwaysThrows should silence missing return hints

[nex3]

When I run the analyzer (2.0.0-dev.7) on the following code:

import 'package:meta/meta.dart';

int foo(bool yes) {
  if (yes) return 1;
  error("no");
}

@alwaysThrows
void error(String message) {
  throw message;
}

I get this message:

hint • This function declares a return type of 'int', but doesn't end with a return statement at test.dart:3:1 • missing_return   

Because the analyzer knows that error() always throws, I'd expect it to be able to deduce that no return is needed, just like it does if I manually write throw "no".

[nex3]

cc @srawlins, who added @alwaysThrows in c6d6a69.

[srawlins]

Fixed with 0a62533.