complete method

void complete ([FutureOr<T> value ])

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 be done at most once.

All listeners on the future are informed about the value.

Implementation 

void complete([FutureOr<T> value]);