You should do this a little differently.  In a node module, `module.exports` is `this`, and in browser it's `window`.  So you can make this file work for both node.js and in browser by doing:

```javascript
(function(root) {
  function VTTCue(startTime, endTime, text) {
    this.id = "";
    this.startTime = startTime;
    this.endTime = endTime;
    this.text = text;
  }

  root.VTTCue = root.VTTCue || VTTCue;
}(this));
```

Now you can add this to the Gruntfile's build setup before vtt.js, and also use it in our FakeWindow.js, but simply `require()`'ing it