connect method
Connects a new endpoint for sessionId. Reusing a live id throws.
Implementation
PdfSyncTransport connect(String sessionId) {
if (_endpoints.any((e) => e.sessionId == sessionId)) {
throw ArgumentError.value(sessionId, 'sessionId', 'already connected');
}
final endpoint = _LoopbackEndpoint(this, sessionId);
_endpoints.add(endpoint);
_broadcastPresence();
return endpoint;
}