your way of asserting inside `then` methods doesn't work.

The way to do right now is to end your promise chain with `.then(done, done)`. This way, the `done` function will get the exception as parameter if the assert fails, and also any other potential errors.

If you had written the test before the code, you'd probably have caught this ;)

When we'll get the new mocha (https://bugzilla.mozilla.org/show_bug.cgi?id=991663) we'll just need to return the promise :)