ES6 class in component script

I would like to use ES6 classes in the components script attribute. When I drop the component onto the canvas the error message "_classCallCheck is not defined" appears.

Component code:

comps.addType('dropdown', {
  model: defaultModel.extend({
    defaults: Object.assign({}, defaultModel.prototype.defaults, {
      'custom-name': 'Dropdown',
      classes: ['c-select2'],
      traits: ['header'].concat(defaultModel.prototype.defaults.traits),
      script: function() {

        class Test {
          constructor() {
            console.log('constructor called');
          }
        }

        let obj = new Test();
      }
    }),
  }, {
    isComponent(el) {
      if(el && el.classList && el.classList.contains('c-select2')) {
        return {type: 'dropdown'};
      }
    }
  }),
  view: defaultView
});
}

Error message:

VM5186:1 Uncaught ReferenceError: _classCallCheck is not defined
    at new asdf (<anonymous>:1:312)
    at HTMLDivElement.<anonymous> (<anonymous>:1:362)
    at <anonymous>:1:8642