Completes future with the supplied values.
The value must be either a value of type T
or a future of type Future<T>.
If the value is itself a future, the completer will wait for that future to complete, and complete with the same result, whether it is a success or an error.
Calling complete or completeError must not be done more than once.
All listeners on the future are informed about the value.
Source
void complete([value]);