Flutter The name FutureOr isn't a type, so it can't be used in an as expression

Question

While upgrading the flutter project to null-safety it has appeared the following error

    The name 'FutureOr' isn't a type, so it can't be used in an 'as' expression.
Try changing the name to the name of an existing type, or creating a type with the name 'FutureOr'

Solution

Looks like import is missing import 'dart:async';

FutureOr is part of dart:async