Add test262 test cases.

[cscott]

This currently only works on node, and only runs Promise-related test cases.

It could be generalized to run a larger subset of the ES2015 test cases, and to not fail horribly when run in a browser.

[ljharb]

I'd love to add all of test262's relevant test cases - but I'm skeptical about "not [failing] horribly when run in a browser" :-)

[cscott]

Well, since the browser doesn't have access to npm, you'd either have to (a) make the test262 package accessible somehow from within the browser, or (b) run some pre-processing step to turn all the test262 test cases into individual mocha tests inside the test directory. Option (b) is probably not too hard, actually.

The other issue with running more of test262 is that they expect each test to have an isolated environment -- ie, you can stomp on a global, and then when you're running the next test things should go back to normal. Mocha doesn't provide that by default. I've deal with this so far by blacklisting the Promise-related tests which stomp on globals; it would be nice to have a better solution.

[cscott]

I should have said that option (c) is just to do a test for typeof process === 'undefined' and if so, bail out of the test case. That would just skip those tests when run in a browser, which would be the minimum thing necessary to "avoid failing horribly".