Recreate 1711 problem
$ npm test
> issue-1711-babel-version@1.0.0 pretest /home/carlerik/dev/sinon-1711-babel
> npm run build
> issue-1711-babel-version@1.0.0 build /home/carlerik/dev/sinon-1711-babel
> babel src -d ./build
src/mod1.js -> build/mod1.js
src/mod1.spec.js -> build/mod1.spec.js
> issue-1711-babel-version@1.0.0 test /home/carlerik/dev/sinon-1711-babel
> mocha build
sinon#stub
✓ should correctly mock the module's method
1 passing (9ms)