This was originally part of bug 1242505, but will land separately after few days.
Fair warning, the mochitest selftests need a bit of extra path hackery to run browser-chrome. I started looking into fixing it but haven't had a chance to finish.
We're actually using a lesser form of self-test here, in that the browser-chrome test runner itself contains the code for detecting failures marked as expected. Thanks for the heads up though!
Pushed by email@example.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/8b0b1a47321a Add self-tests for unhandled rejection handling in browser-chrome tests. r=Mossop