Support default parameters in ES6 and moz

[MikeRatcliffe]

Default parameters are currently not supported and using them completely invalidates the linting from that line onwards. For Firefox devs this means that it is often only possible to lint the first few lines of a file.

Try linting this:

function Doit() {
}

Doit.prototype = {
  _someProperty: null,

  test: function(num, num2=1) {
    return num === num2;
  },

  get someProperty() {
    return this._someProperty;
  },
};

It would be awesome if support could be added for this feature in ES6 and moz modes.