Some history:
* `uglifyjs-webpack-plugin` < v1.0 used the minifier `uglify-js`
* however `uglify-js` does not support ES6, which resulted in a fork
called `uglify-es` that was developed in the `uglify-js` repository,
but under the `harmony` branch
* `uglifyjs-webpack-plugin` v1.x switched to `uglify-es` for ES6 support
* however `uglify-es` stopped being maintained:
https://github.com/mishoo/UglifyJS2/issues/3156#issuecomment-392943058
* which led to a fork called `terser` that has incorporated all of
the unmerged PRs and will be where all new development occurs:
https://github.com/fabiosantoscode/terser
* `terser-webpack-plugin` was created, which is the `terser` equivalent
of `uglifyjs-webpack-plugin`:
https://github.com/webpack-contrib/terser-webpack-plugin
* `uglifyjs-webpack-plugin` v2.x will be switching back to `uglify-js`,
so any project that needs to support ES6 now needs to switch to
`terser-webpack-plugin`.
Fixes#7923.
Ed Morley
Florent Cailhol