@:structInit class properties cannot be marked optional on some targets

When marking a class as `@:structInit`, "non-real" properties are required during creation.

Marking the property with `@:optional` will work around the issue for some targets, but not others. Notably, Hashlink will throw a runtime exception, and Eval will give you a diferent, more exciting compiler error.

Try.haxe url: https://try.haxe.org/#AE0CAbd0

full example, for posterity:
```
@:structInit
class StructTest
{
public var normalVar: Int = 5;
@:optional
public var prop(get, set): Int;

public function get_prop()
{
return 10;
}
public function set_prop(v)
{
normalVar = 10;
return v;
}
}

class Test {
static function main() {
var empty: StructTest = {};

trace( empty.prop );

}
}
```
JS/Neko: Works.
HL: Compiles, throws an exception during execution: `StructTest does not have field prop`
Eval: Fails to compile: `Field index for prop not found on prototype StructTest`