close method
void
close()
Close a mailbox.
If the mailbox has a message we leave the message intact so that it can be read by the consumer.
Implementation
void close() => _mutex.runLocked(() {
assert(_mailbox.ref.magic == _magic, 'Mailbox memory corrupted/UAF');
_mailbox.ref.open = _stateClosed;
_condVar.notify();
});