text: const msg = 'Using ES6 features.<br/>';
  
  document.write(msg);
  
  class Animal {
    constructor(name) {
      this.name = name;
    }
  
    speak() {
      document.write(this.name + ' makes a noise.');
    }
  }
  
  class Dog extends Animal {
    speak() {
      document.write(this.name + ' barks.');
    }
  }
  
  var d = new Dog('Mitzie');
  // 'Mitzie barks.'
  d.speak();
  