private method
Returns a private BroadcastChannel for name.
The driver will prefix the name (e.g. 'private-') as required by the
server and perform the appropriate auth handshake. Like channel, a
repeated call for a registered name must return the held instance
until a leave or a disconnect: AuthChannelSubscription calls this on
every sync and takes a different instance to mean the channel was left.
Implementation
@override
BroadcastChannel private(String name) {
final String prefixed = 'private-$name';
return _handles[prefixed] ?? _hold(_FakeBroadcastChannel(prefixed, this));
}