Hi,
At Finaps we have developed a new development environment to create Mendix widgets which include libraries like React. It is actually an adaptation of a NPM, Webpack, and Babel toolchain, so you can easy include any NPM package and target (very) old browsers like for instance IE 9. This new toolchain works for any Mendix version which we've tested (that is Mx 6 and 7) and is future-proof.
This isn't actually a question, we just want to share this with the community because we are really excited about the new possibilities for widget development! It is really cool to be able to use technologies like React, Plotly, and Material Design components. The boilerplate, with an explanation on how it works, can be found for free (open-source license) at:
https://github.com/Finaps/boilerplate-mendix-react-widget
Have fun developing widgets! We really like to hear your experience with widget development :)