stable and es6 code version produce different output

Where did the code come from?

From trying to shrink js code.

Your code snippet does not produce an error, that made me stumble over this and thinking there is a special implementation of arrow functions.

copy & paste WebIDE left side:

>var _b, b = () => {_b = this;};
=function (undefined) { ... }
>new b()
=b: {  }
>