Arrow function doesn't correct.

[xgrommx]

Hello @sebmck I use babel and arrow function like this:

let x = () => {
  console.log(this);
};

x();

But in console.log output undefined. This is very important problem in my project.

[sebmck]

See the FAQ, also a duplicate of #1700, #1667, #1573, #562, #814, #730 and #733.