I have not seen anything that specifically says so either. On the other hand, I'm not sure why I'd want to feed my dogs leeks anyway
So in my case at least, it becomes a moot point.
Having said that, whilst leeks are a member of the onion family, I believe they're a weak member (much as garlic is). In small quantities, I would not expect them to do a great deal of harm. As such, I wouldn't panic unduly if my dog happened to eat some, but at the same time, wouldn't deliberately feed them either (unlike garlic, I'm not aware of any particular properties of leeks that are desirable either - so don't see any point in giving them).