Possible Feature: Optionally don't remove references

[pzuraq]

This just popped up in ember-decorators/argument#43, we have a use case for wanting to be able to just remove the imports, and keep all references.

Basically we have a shim that acts just like the global class Element in the browser (just re-exports it), but exports a shim class to make fastboot work in Node environments. When we strip all the layers of type checking away the browser code should work exactly the same as before, so there is no need to remove the references.

I'm currently adding a plugin to the library itself to do this, I'm not sure it's going to be a very common use case at all so I definitely understand not adding it upstream here, but I thought I'd bring it up.

[layershifter]

@pzuraq feel free to open PR 👍