ClosureAttribute2Optional

@propertyWrapper final class ClosureAttribute2Optional<A0, A1, ReturnType> where A0 : ConstructibleFromJSValue, A0 : ConvertibleToJSValue, A1 : ConstructibleFromJSValue, A1 : ConvertibleToJSValue, ReturnType : ConstructibleFromJSValue, ReturnType : ConvertibleToJSValue