istanbul ignore babel compilation sideeffects

As you can see its currently impossible to achieve 100% coverage for .js.es6 files.

We can add istanbul inline ignore comments to help this. For example:

(function () { 'use strict';
 
var _createClass = (function () { ... })();

/* istanbul ignore next */
function _classCallCheck(...args) { ... }
 
(function () {
  var global = window.gl || (window.gl = {});
 
  var SomeClass = (function () {
    /* istanbul ignore next */
    function SomeClass() {
      _classCallCheck(this, SomeClass);
    }

...

This tricky part is that this is compiled by sprockets-es6, so we would likely have to contribute upstream.

https://github.com/gotwarlost/istanbul/blob/master/ignoring-code-for-coverage.md

@winniehell