Support ES6 coverage

[pjasiun]

Coverage of a ES6 transpiled code does not display properly, because babel adds some additional lines which are not covered.

We could try to updated istanbul version and check if it helps. We could also try https://github.com/douglasduteil/isparta to show the coverage in the ES6 files, not transpiled.
Also, if it does not help we could decide whether we want to keep using istanbul, or there is a better project.