current property
The current value of the stream.
Is null before the first call to moveNext and after a call to
moveNext completes with a false result or an error.
When a moveNext call completes with true, the current field holds
the most recent event of the stream, and it stays like that until the next
call to moveNext.
Between a call to moveNext and when its returned future completes,
the value is unspecified.
Implementation
T get current;