isOpen property
Whether the channel can still carry bytes: false once destroy has run, or once this side has closed it.
A far-end close turns it false only where the transport can see one: the
WebSocket transport reads its socket's own state, while
SecureSocketTransport sees only a local close. inbound ending is the
signal every transport gives, so a consumer that must notice the far end
going away watches inbound.
Implementation
@override
bool get isOpen => _open;