Omit code for optional features that are turned off

Talked to @pzuraq and he mentioned that it's theoretically possible to strip out ember-source code that won't be used due to optional-features. This tooling would be great incentive for apps to invest in moving in the direction the core team/communities deems as the way forward.

Not sure where this would happen, but opening here since it seems like a build time concern

(Reopening https://github.com/ember-cli/ember-cli/issues/9021)