React: Modernize build process / tooling

[zpao]

This is mostly a prereq of #6655.

Goals:

• align build tooling (right now mixing grunt & gulp - commit, probably to gulp #4927)
• add watching, should be fast to rebuild and test changes in browser
  
  
  
  • could be something like webpack hot reloading
  
  
• move shared tooling back into fbjs (babel plugin, custom lint rule)

Non-goals but things to keep in mind:

• alternate build approaches, in particular flat bundling - it needs to be easy to swap this in
• releasing from master - we may want to have development happen such that feature flags are used to turn on/off features. we may want the build result to be a static flag set (no runtime checks) but would need to be possible to target any flag combination easily

[trueadm]

The work for this has been merged in PR: https://github.com/facebook/react/pull/9327

[gaearon]

Not sure, I think these are open areas:

• add watching, should be fast to rebuild and test changes in browser
• move shared tooling back into fbjs (babel plugin, custom lint rule)

[gaearon]

We got rid of fbjs and actually seem to prefer to own our scripts.
As for watching.. seems like it might be complicated with our pipeline. Maybe not worth it.