I had the same question, and without actually cloning the repo I wish I had a better answer.

I found, looking through the tags, that src/React.js was added in 0.14-stable, then I examined the history of React.js to find this commit:

https://github.com/facebook/react/commit/b2ca3349c27b57b1e94...

Additionally, I dug this up:

https://github.com/facebook/react/pull/4814#discussion-diff-...

https://github.com/facebook/react/pull/4814

"I changed it to __SECRET_DOM_DO_NOT_USE_OR_YOU_WILL_BE_FIRED to make our German friends less grossed out. I'm going to do the examples in a separate PR."