Complete future with an error.
Calling complete or completeError must not be done more than once.
Completing a future with an error indicates that an exception was thrown while trying to produce a value.
If error is null, it is replaced by a NullThrownError.
If error is a Future, the future itself is used as the error value.
If you want to complete with the result of the future, you can use:
thisCompleter.complete(theFuture)
or if you only want to handle an error from the future:
theFuture.catchError(thisCompleter.completeError);
Source
void completeError(Object error, [StackTrace stackTrace]);