invalid assignment through `untyped` causes assertion fail

```haxe
class Test {
static function main() {
var foo = new Foo();
untyped foo.bar = function() trace('bar');
foo.bar();
}
}
class Foo {
public function new() { }
public inline function bar() trace('foo');
}
```
Results in assertion fail at https://github.com/HaxeFoundation/haxe/blob/f2f4dc15144c8c76597c1ec9dac66a0b23b5dcc7/src/typing/typer.ml#L514-L515
Without untyped it properly gives error message. Should this assertion be changed into error or should it be handled sooner?