I've now got the setup to run Mocha tests through SauceLabs, and to answer the question, `ready` bubbles to `window` for everyone except IE8 (none of use are surprised by that :poop:):

https://saucelabs.com/u/patheard

Here's [the project](https://github.com/patheard/saucelabs) I created to get the above results.  It's based on this [handy tutorial](https://saucelabs.com/docs/javascript-unit-testing-tutorial) (and comes with a whole [tonne of browsers/platforms](https://saucelabs.com/docs/platforms/webdriver) we could run against).