That really disturbed me during the review, it is hard to say what is going to be broken due to the non-existance of these variables.
It looks like filesystem API are only used for `@import` inlining. I tend to think we should just remove these lines and throw an exception around here https://github.com/mozilla-b2g/gaia/pull/8954/files#L0R396 to say that we do not support `@import` when optimizing. Then we would be able to remove `_rebaseRelativeURLs` and code in `_inlineImports` that is using fs/path/existsSync.

If we do that, then we can assume that all the code from clean.js is supposed work.