Unexpected syntax error when declaring a class inside another class

class Tester {
  constructor() {
    new (class {})();
  }

  static get test() {}
}

This block of code will throw a syntax error, even though it is valid ES6.

Expected token `('
       static get test() {}

It also does not error when I write directly in the QML view, only when I import it from a Javascript file.