Clay is absolutely right - that zoo should know better than that.
Some people will say "Oh, I've mixed species in tanks before and never saw any problems". Well, with dart frogs, you can't always tell if there's a problem until you start finding them dead in the tank. And that zoo cannot possibly recreate the correct enviornment for both the dart frogs and the snake in the same tank, therefore one or both species will not thrive. And even if there's no chance of that snake eating any of the frogs, I'm
sure the snake being in there is extremely stressful to them.
Also, since they are keeping darts with a snake, I'm sure it's not just one type of dart frog in there, which is also a big no no. It's a bad idea to keep different dart frogs together, even if they are the same species but different morphs. For example, you shouldn't keep blue D. auratus with green D. auratus. Why? Because they can interbreed and produce hybrids, and we are trying to retain the integrity and purity of the species.
Short answer:
Please do not mix dart frogs with different species or morphs of other dart frogs, and
definatly do not mix dart frogs with any other animal.
The only "benefit" of doing something like that is your own personal viewing pleasure.
Please think of the animals, and what's best for them.
In the case of dart frogs, what's best for them is to keep them with their own species and morph.
If you would like more information on this topic, please read this:
http://www.dendroboard.com/forum/beginner-discussion/3449-mixing-multispecies-exhibits.html