didExceedDeadline method Null safety
protected">@protected
Override to be notified when deadline is exceeded.
You must override this method or didExceedDeadlineWithEvent if you
supply a deadline. Subclasses that override this method must not
call super.didExceedDeadline()
.
Implementation
@protected
void didExceedDeadline() {
assert(deadline == null);
}