SyntheticEvent.augmentClass undefined in QtWebKit browsers

[hotboi64]

Do you want to request a feature or report a bug?
bug

What is the current behavior?
Everything seems terrible.
In a certain WebKit browser, https://github.com/annulen/webkit, a page that pulls in a bundled script doesn't seem to finish loading, with console errors about augmentClass being undefined

Build details:
webpack-ing,
using babel-loader with es2015 and react presets in webpack.config.js

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via https://jsfiddle.net or similar (template: https://jsfiddle.net/84v837e9/).
After building,
use a QtWebKit test browser from ./WebKitBuild/Release/bin/QtTestBrowser to visit the offending page

What is the expected behavior?
to not see complaints about SyntheticEvent in the console

Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?
react and react-dom 15.4
QtWebKit 5.212

[walrusfruitcake]

Upon debugging just before the SyntheticEvent module export, it appears SyntheticEvent.augmentClass is undefined, but the SyntheticEvent.Interface assignment is EventInterface as we'd expect.

[walrusfruitcake]

#10011

[gaearon]

Fixed by #10011.
But no promises this won't break again.