I don't think this is going to work properly since this will compile to:

```js
var WeakMap;

WeakMap = WeakMap || require('harmony-collections').WeakMap;
```

So the WeakMap var will override the global var and the require will always be run.

I think instead you want something like:

```coffee
WeakMap = global.WeakMap ? require('harmony-collections').WeakMap
```