move uglify-js to dependencies

[kellyselden]

Closes #1391

[nknapp]

Personally, I'm in favour of keeping the dependency optional and handle the missing library case. It greatly reduces the amount of dependencies and downloaded megabytes if you don't need the precompiler. Opinions?

[rwjblue]

Agreed, but with the move to ES modules how do we conditionally import?

[nknapp]

I would make an execption here. It may make sense to move to completely towards es6-modules, but if they lack support for essential features, we should omit them when we can't use them.

I would suggest using require('uglify-js') instead and adding a lengthy comment, with the reason for not using es6.

[nknapp]

I made a PR here: #1394

[nknapp]

I'm closing this in favor of #1394