Remove unused code from production build

[aweary]

See facebook/react#9398

Can we remove junk code from PropTypes prod build? It doesn't use bundle-collapser so it ships all those method names (e.g. “fbjs/lib/invariant”). This is fixable if you add “-p bundle-collapser/plugin” to browserify call. Additionally, there seems to be some junk invariant() code which is easy to fix if we just turn this into an inlined error. https://unpkg.com/prop-types@15.5.6/prop-types.min.js

[gaearon]

Want to help? @aweary

[aweary]

@gaearon happy to!

[aweary]

Resolved by #20