add<T> method
Adds new subscription to this composite.
Throws an exception if this composite was disposed
Implementation
StreamSubscription<T> add<T>(StreamSubscription<T> subscription) {
if (isDisposed) {
throw StateError(
'This $runtimeType was disposed, consider checking `isDisposed` or try to use new instance instead');
}
_subscriptionsList.add(subscription);
return subscription;
}