Minor See https://github.com/browserslist/browserslist
We could use some query like "last 2 versions, not dead, firefox ESR".
To configure browserlist in the package.json file, we need to migrate to babel 7, so we need to drop node.js 4 support
=> that means ES5 build will be used only on browsers.
As the browser build or the ES5 build will use the same babel configuration and browserlist, we can assume that if tests pass for the browser build, the ES5 build is also OK.
