close method
      
void
close()
      
     
    
Close a mailbox.
If mailbox already contains a message then close will drop the message.
Implementation
void close() => _mutex.runLocked(() {
      if (_mailbox.ref.state == _stateFull && _mailbox.ref.bufferLength > 0) {
        malloc.free(_mailbox.ref.buffer);
      }
      _mailbox.ref.state = _stateClosed;
      _mailbox.ref.buffer = nullptr;
      _mailbox.ref.bufferLength = 0;
      _condVar.notify();
    });