I'm a little divided on using `chai`. I mean,

* we have a lot of custom asserts anyway.
* mocha docs say it can take anything that throws errors.
* an assert library doesn't have to do much in our case, or in most cases, really.

So we're probably not saving all that much code. I admit that I don't know `chai` all that well, so maybe there are other benefits that I'm not seeing. Maybe the mocha integration isn't as simple as the docs say?