As per the Dart documentations:

A metadata annotation begins with the character @, followed by either a reference to a compile-time constant (such as deprecated) or a call to a constant constructor.

Nevertheless, consider the following excerpt from the Flutter source code:

void handleTapDown({ required PointerDownEvent down });
//                   ^^^^^^^^

If it's a metadata annotation, then why it doesn't begin with the prefixed @ sign? If it's not, then what is it?!