PosInfos in accessors?

Just noticed that `PosInfos` doesn't work on accessor methods. I guess they should work...?

```haxe
class Test {
var a(get, null) : String;

function get_a(?pos : haxe.PosInfos) {
return a;
}

function new(a) this.a = a;

static function main() {
trace(new Test("12345").a);
}
}
```

```
Test.hx:4: lines 4-6 : In method get_a required by property a
Test.hx:4: lines 4-6 : ?pos : Null -> String should be Void -> String
```

Also available at http://try.haxe.org/#60A1d