Add DefinePlugin for NODE_ENV on Real World Example

[willrstern]

NODE_ENV=production npm start in the Real World Example always loads Root.dev component (which includes dev tools), because NODE_ENV is never set on the client:

• add webpack DefinePlugin for process.env.NODE_ENV

[gaearon]

This is partly intentional because just adding DefinePlugin will not be enough for a real production setup, and we might give a false impression by providing a half-working setup. This is why I opted for providing no production setup in examples at all.

I plan to fix this as part of #1800 but it will take some time. In the meantime, I would rather not introduce inconsistencies.

Thank you for taking time!

[willrstern]

Ah, didn't know that! Thanks @gaearon for taking the time to explain. Sorry to take up repo-time on that.