Remove references to `window`.

These references are already setup properly using `this` which will work even in non-browser environments or when we do not want to be working in globals mode (for example when supplying `Ember.lookup` or `Ember.exports` as mentioned in https://github.com/emberjs/ember.js/commit/305202f).

This allows us to set the stage for CJS builds of ember-metal and ember-runtime (still needs more work in transpiler and tests).