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

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/b2ca3349c27b57b1e9462944cbe4aaaf76783d2b https://github.com/facebook/react/commit/b2ca3349c27b57b1e94... Additionally, I dug this up: https://github.com/facebook/react/pull/4814#discussion-diff-39007098 https://github.com/facebook/react/pull/4814#discussion-diff-... https://github.com/facebook/react/pull/4814 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."