cancel abstract method Null safety
Cancels the stream iterator (and the underlying stream subscription) early.
The stream iterator is automatically canceled if the moveNext future
completes with either
false or an error.
If you need to stop listening for values before the stream iterator is automatically closed, you must call cancel to ensure that the stream is properly closed.
Returns a future which completes when the cancellation is complete. This can be an already completed future if the cancellation happens synchronously.