Don't assume window is defined when polyfilling Promise

[pugnascotia]

I know server-rendering isn't supported, but I was tinkering with loading the production bundle in Nashorn, Java 8's JavaScript engine, and found that the Promise polyfill assumes that window exists. This PR changes the polyfill to attempt to use window first, and global secondly.

For the interested reader, global isn't defined in Nashorn either, but it can be easily defined in order to bring it in-line with Node.

Testing steps:

npm run create-react-app mytestapp
cd mytestapp
npm start
# App starts OK and renders in the browser

npm run build
# App builds successfully.

# Also need to change generated src/index.js to only mount React if document is defined
vim src/index.js
# Start Nashorn interpreter
jjs
> var global = {}
> load('build/static/js/main.26610bf4.js')
# File load without throwing an exception

[gaearon]

loading the production bundle in Nashorn

Can you describe the use case?
I’m worried that even if we take this, we might break it again in the future since we don‘t support SSR.

[pugnascotia]

The use case? That would be...SSR :-) It was the only library-side change I had to make to get it to load in Nashorn, so I thought it was benign enough, but I do take the point about it not being supported.

[gaearon]

How do you know which file to execute? Are you using the generated manifest?

[pugnascotia]

I haven't gotten that far yet but yes, that would be the obvious thing to do.

[gaearon]

I think I won’t take it as is for the reasons above.
But I’m open to review it as a part of something like #1100.

I’d like to approach this as a holistic feature so we understand how to keep it working and extent to which we support this, and don’t break it in the future.

Thanks!