Add 'DEV' flag

[flarnie]

We want to have a check for whether React is in 'DEV' mode, which would work for all previous versions of React.

[gaearon]

which would work for all previous versions of React.

I think this is a non-goal (as it's something we'd implement on DevTools side with duck type checks etc). Just providing a boolean as an argument to the inject call should be enough.

[gaearon]

One thing we need to ensure is it highlights in DEV mode even if React thinks it's in prod mode. For example if you set process.env.NODE_ENV to "production" but don't uglify (and thus get deoptimizations caused by code in DEV blocks).

[gaearon]

Also the opposite: it should show as DEV if you uglify but don't set process.env.NODE_ENV to "production". Tricky 🤔

[gaearon]

I think this was fixed in #9586, and we won’t be bringing it to 15.x.