call method
Dispatch a payload to this event stream. This only works if
key
is the correct key with which this Event was constructed.
Implementation
void call(T payload, DispatchKey key) {
if (key != _key) {
throw ArgumentError(
'Event dispatch expected the "${_key.name}" key but received the '
'"${key.name}" key.');
}
_streamController.add(payload);
}