Babel 6 bug with default arguments in classes

original.js

class Z {
  x(y = 2) {
    return y
  }
}

transpiled.js

"use strict";

var _arguments = arguments;

var _createClass = (function () { function defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } } return function (Constructor, protoProps, staticProps) { if (protoProps) defineProperties(Constructor.prototype, protoProps); if (staticProps) defineProperties(Constructor, staticProps); return Constructor; }; })();

function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }

var Z = (function () {
  function Z() {
    _classCallCheck(this, Z);
  }

  _createClass(Z, [{
    key: "x",
    value: (function () {
      function x() {
        // This should be using arguments!
        var y = _arguments.length <= 0 || _arguments[0] === undefined ? 2 : _arguments[0];

        return y;
      }

      return x;
    })()
  }]);

  return Z;
})();