Trying to understand behavior of globals when loading tests

In the process of setting up Karma to test our frontend, I ran into issues getting [sinon.js](sinonjs.org) to work correctly. Sinon relies on detecting the existence of `global.XMLHttpRequest` to stub out their fake HTTP Server.

It turns out that something in the Karma + PhantomJS stack normally sets `global.XMLHttpRequest`. But if an imported JS file tries to set globals itself by using the following snippet, it's no longer set.

```
if (window.global == null) { // Generated by coffeescript's ?= operator
window.global = {};
}
```

I suspect I may be missing a detail on the behavior of window vs global.

I'm trying to understand when does `global.XMLHttpRequest` and the rest of global get set during normal initialization? By which tool (Karma or PhantomJS)? And why does that not happen before files are loaded?

You can see a reproducible example here: https://github.com/sean-lynch/karma-global-test
### Expected behavior
- `global.XMLHttpRequest` and other globals would be set before files are loaded, as it would be when loading into a browser
### Actual behavior
- Not set if another file defines a global object while loading.
### Enviroment Details
- Karma version (output of `karma --version`): 0.13.22
- Relevant part of your `karma.config.js` file

files: [
// Fix to solve problem with window.global not being set properly
//{ pattern: 'global.js', watched: false },

// Then load common
'common.js',

// Load project files that are test ready
'test.js'
],
### Steps to reproduce the behaviour
1. Clone https://github.com/sean-lynch/karma-global-test
2. Run `npm install`, then `npm run test` - See that it fails because global is not set
3. Try commenting out `common.js` in the files section of karma.config.js and see that it's fixed.