This repository is a running example of how to configure a project for testing Babel transpiled ES6 JavaScript that uses ES6 module syntax, using Mocha, Sinon and Proxyquire without adding any dependency to whatever module bundler/loader is used for delivering the code to browsers.
I've tested this in node 6.x and 7.x. If you're using ES6 modules, then you probably also have a fairly modern node on your system(s).
$ npm install
$ npm test