Fix UMD bundles, making safe to use as required modules

[zpao]

I think this fixes the problem. There's a giant comment in there explaining why and how.

I want to stress that this is a short-term solution. It works and the reasoning is sound but I'm not happy that we need this. It's probably time to start using webpack to bundle but that would have taken me much longer to go do.

Byte-wise this should be fine. We're just adding the code that you can see in there. Since we do this after the bundle process is done, that require('react') isn't analyzed.

Todo:

• Look for any other expected globals in shims
• Use this in other bundles
• Look into the RequireJS situation. I honestly don't care much about that but we should probably make it work. It would require rejiggering the wrapper (since we couldn't execute the UMDed module immediately)

cc @facebook/react-core

Fixes #7482

Test Plan:

• Made a hello world app with webpack and aliased react and react-dom.
• Ran the same hello world using the dist files directly (so React & ReactDOM were globals).

[zpao]

The other case we have is ReactWithAddons, which has a shim that uses a global ReactDOM. However we've never supported using ReactWithAddons as a drop-in replacement in a require system. It should continue to work fine when using globals (eg script tags) just not replacing require('react/addons'). We'd get into a circular dep situation.

[zpao]

Ok, got this working and tested for ReactDOM/Server. I haven't tested the AMD stuff (meh) but I think it's right now. @gaearon gave me the thumbs up so I'm going to just merge and get 15.4 RC out.

[aweary]

It became really what?! 😄

[zpao]

Too late! I pressed merge.

It became really "hard". Or maybe I wrote "annoying"? I'm going to leave the mystery in there for now :)

[gaearon]

I think this should be define, not require.

[zpao]

I think what I have is right. define is used to define the exports, which isn't what we get back from calling f. require just makes sure the module is available and runs code without exports. Same reason we don't do module.exports = f(require('react') for the CommonJS case. It would be define in a UMD wrapper and that is what browserify will build and that we call inside f.

[gaearon]

Looked in the docs and you're right. 😅