Babel 编译 ES6 的 class extends Array 时，无法获取方法，求助

代码如下

 class MyCoolArray extends Array { first() { return this[0];} } var a = new MyCoolArray(1,2,3); console.log(a.first());// get error 'a.first is not a function' 

谁知道为什么 first 无法执行？谢谢

babel 在线 https://babeljs.io/repl/#?babili=false&evaluate=true&lineWrap=false&presets=es2015%2Ces2015-loose%2Ces2016%2Ces2017%2Clatest%2Creact%2Cstage-0%2Cstage-1%2Cstage-2%2Cstage-3&code=class%20MyCoolArray%20extends%20Array%20%7B%0A%20%20first()%20%7B%20return%20this%5B0%5D%3B%7D%0A%20%7D%0A%0Avar%20a%20%3D%20new%20MyCoolArray(1%2C2%2C3)%3B%0A%0A%0Aconsole.log(a.first())%3B