error stack should be displayed for uncaught exceptions in async tests

[janotav]

Currently exceptions in async tests lose their stack and display unfriendly message like:

Chrome 28.0.1500 (Linux) simple test FAILED
    Error: Uncaught AssertionError: expected 1 to equal 2 (/home/janotav/anywhere/web-ide/client/core/components/chai/chai.js:505)
        at global.onerror (/home/janotav/anywhere/web-ide/server/node_modules/mocha/mocha.js:5343:23)

This has been addressed recently in the HTML5 spec and can produce nice output like:

Chrome 31.0.1605 (Windows 7) simple test FAILED
    AssertionError: expected 1 to equal 2
        at Assertion.assertEqual (/home/janotav/anywhere/web-ide/client/core/components/chai/chai.js:907:12)
        at Assertion.ctx.(anonymous function) [as equal] (/home/janotav/anywhere/web-ide/client/core/components/chai/chai.js:3113:25)
        at /home/janotav/anywhere/web-ide/client/tests/spec/idb-filesystem-spec.js:43:34

If the global handler in mocha re-uses the existing Error object like this:

    global.onerror = function(err, url, line, colno, erroObj){
        fn(erroObj || new Error(err + ' (' + url + ':' + line + ')'));
    };

Currently this extra arguments work in Canary, other browsers are likely to follow. Related blog post:

https://mikewest.org/2013/08/debugging-runtime-errors-with-window-onerror

[boneskull]

see #815

[mgiuffrida]

This wasn't resolved with #815. We should still include the full stack trace of uncaught exceptions if provided, rather than throw out the error and create a new one with no context.

Repro: https://jsfiddle.net/mgiuffrida/koLzuy6s/

function doSomething() {
  chai.assert.equal('foo', 'bar');
}
describe('mocha', function() {
  it('throws synchronously', function() {
    doSomething();  // Stack trace OK
  });
  it('throws async', function(done) {
    setTimeout(function() {
      doSomething();  // Truncated stack trace
      done();
    });
  });
});

Unhelpful error (no function stack or real line/col numbers):

Error: Uncaught AssertionError: expected 'foo' to equal 'bar' (https://fiddle.jshell.net/mgiuffrida/koLzuy6s/show/:278)

Helpful error (with https://github.com/mgiuffrida/mocha/tree/patch-1):

AssertionError: expected 'foo' to equal 'bar'
    at doSomething (:52:17)
    at :61:9