support environments with both window and global

Such environments include [node-webkit](https://github.com/rogerwang/node-webkit) and [atom-shell](https://github.com/atom/atom-shell).

This breaks the following tests within test/unit/node/generated-code-dependencies.js:
* `test('class'`
* `test('generator'`
* `test('generator (symbols)'`
* `test('compiled modules'`
* `test('compiled modules inline'`
* `test('compiled modules mix inline & script'`

I have not yet investigated the `child_process.exec()` based tests, however the `vm.runInNewContext()` tests have an issue with `global` within indirect `eval()`s. It gets pretty deep and difficult to debug. I am still working on this, but any help would be appreciated.
Thanks!! Tests pass now!

Listed `window` first, though, as it's the most parent scope in node-webkit with `window.global`.