Add optimize-js-plugin to webpack.config.prod.js

[ggregoire]

This PR adds the optimize-js lib to the build process:

• https://github.com/nolanlawson/optimize-js
• https://github.com/vigneshshanmugam/optimize-js-plugin

Benchmarks from @nolanlawson:

Browser	Typical speed boost/regression using optimize-js
Chrome 55	20.63%
Edge 14	13.52%
Firefox 50	8.26%
Safari 10	-1.04%

[gaearon]

Does(n't) Webpack 2 do it by default? Just asking since we'll move to it soon.

[ggregoire]

I honestly have no idea. This comment makes me think that it doesn't do it.

[gaearon]

Need to wait for this until vigneshshanmugam/optimize-js-plugin#7 is fixed since we're moving to Webpack 2 soon.

[nolanlawson]

As the author of optimize-js, I would not recommend it as a general-purpose tool for something like create-react-app.

Because optimize-js targets the existing behavior of JS engines using heuristics (one might say "hacks" even 😉) and because this behavior varies depending on the shape of the JS bundle, it's essential for users to run their own measurements (in multiple browsers, ideally!) and determine if optimize-js is really an improvement. For some bundles it may actually make things slower, and for Safari/JSCore most of my tests show it's basically a wash.

In other words, "ask your doctor if optimize-js is right for you." 😃

[gaearon]

Fair enough, thanks!