Assign to self

`isomorphic-fetch` will need to be changed to stub `global.self` instead of `global.window`.

https://github.com/matthew-andrews/isomorphic-fetch/blob/master/server.js

Changes #48.
Obsoletes #49, #50.

cc @dgraham @matthew-andrews @knowledgecode
Oh nice, I didn't realize there was a `worker` option.

I'll give `exnext` a try too. Last time I think I was on an older jshint that didn't define `Promise` yet.