More explicit window context.

Hi,

I'm adding coffee-script support for a node-webkit application. When it comes to node or window context, things can get quite weird over there:
  https://github.com/rogerwang/node-webkit/wiki/Transfer-objects-between-window-and-node

When loading `browser.coffee` in the (`node-webkit`) browser, a couple of exceptions were raised at places where an explicit `window` context was needed.

I've fixed them with this PR. I figured it might be interesting for others..