Use webpack IgnorePlugin to exclude 'react-is' from production build (#6084)
Use webpack IgnorePlugin to exclude 'react-is' from production build (#6084)

`react-is` isn't used in production, so we shouldn't bundle it.

Note: most of those plugins are using the `dev` variable, but in case someone runs `NODE_ENV=development next build`, they would need a copy of `react-is` because the conditional use of `react-is` checks `NODE_ENV` — not whether or not HMR is being used (what what the `dev` variable is based on).
flybayer/next.js,JeromeFitz/next.js,azukaru/next.js,azukaru/next.js,BlancheXu/test,JeromeFitz/next.js,flybayer/next.js,azukaru/next.js,JeromeFitz/next.js,flybayer/next.js,zeit/next.js,JeromeFitz/next.js,zeit/next.js,azukaru/next.js,flybayer/next.js,BlancheXu/test,BlancheXu/test,zeit/next.js