Updates mocha.js to address #1700

[eddies]

Fixes #1700

[dasilvacontin]

Hi @eddies! Could you provide tests that pass thanks to this change? Cheers!

[eddies]

@dasilvacontin Honestly, I'm not sure how to provide a unit-like test for this. I was only triggering this issue as part of running a test w/ PhantomJS and Grunt and and various other dependencies. I did create https://github.com/eddies/mocha-test-case as a minimal example, which I believe definitively isolates the error to the specific one-line commit to mocha I highlighted.

If someone could take a look at the mocha-test-case repo and provide some suggestions as to how that might be translated into a test for the mochajs repo, I could try running with it from there.

[danielstjules]

@eddies Please make the change against runnable.js, rather than the compiled/disted mocha.js.

mocha/lib/runnable.js

Line 215 in 49d81aa

// Discard the resolution if this test has already failed asynchronously

Beyond that, how does this impact #1578 and #1540 ? After updating runnable.js, I wonder if the build fails.

Edit: It looks like the tests continue to pass

[jbnicolai]

Picked up @danielstjules's comment and merged in a separate branch.