moveNext abstract method Null safety
Wait for the next stream value to be available.
Returns a future which will complete with either
true means that another event has been received and
can be read as current.
false means that the stream iteration is done and
no further events will ever be available.
The future may complete with an error, if the stream produces an error,
which also ends iteration.
The function must not be called again until the future returned by a previous call is completed.