Fix issue for running under node-webkit.

[tavwizard]

Under node-webkit we have both contexts 'window' and 'global'.
'window' is default context then we should check for window first.

[ljharb]

This was changed from window to self for web workers in 4e4dc69.

We may need all three.

Also, please include a test that fails without your change, or, indicate in the PR which tests failed.

[tavwizard]

I think we can use self.
I don't know exactly how to test it in mocha.

Under node-webkit we have global and window context. Scripts running under window.
Result: es6-shim will patch global instead of window.

[ljharb]

@david-risney, can you weigh in on your change in 4e4dc69 versus this PR?

[david-risney]

I changed window to self for web workers as you note because in the browser window window.self === window and in the browser worker the global object is named self. So self works for both browser window and browser worker. Neither global nor window exist in a browser worker so changing this back to window will break workers again.

[ljharb]

In other words, we may need a more reliable way to get to the global object than just checking for "global", "self", or "window". Thanks for confirming!

[david-risney]

A common pattern I see elsewhere obtains the global object generically like in es5 via checking this in the global context:

function(global) {
    // ... Everything goes here
})(this);

Afaik this is a feature of JavaScript regardless of use strict or the execution environment (browser, worker, node etc)

[ljharb]

In strict mode, "this" when not assigned is undefined, and does not refer to the global object.

[david-risney]

IIRC that's true generally but not for global code. In global code this is still the global object. http://stackoverflow.com/a/6483866