[js] reserved field name conflicts

```haxe
class Main {
static var __name__ = "hello";

static function main() {
var o = {__class__: "hi!"};
trace(Type.typeof(o).match(TClass(_))); // true
Type.getClassName(Main); // FAIL
}
}
```

I think we should use `$`-prefixed names instead, so we don't conflict with valid haxe identifiers.