Setter not triggered with abstract

In some cases it seems doing a set inside an abstract does not trigger the value setter.

```haxe
enum E {
A;
}
class Main {
public static var flags(default,set) : haxe.EnumFlags = new haxe.EnumFlags();
static function set_flags(v) {
throw "Not reached";
return flags = v;
}
static function main() {
flags.set(A);
trace("Done");
}
}
```