setTimeout with lambda causes incorrect minification

When I use arrow function:

` setTimeout(() => this._check(), 1000);`

it completely messes up the minified scripts.

My widget that contains that function ends up like this:

```
NEWS.CheckIfItemExists.prototype,
this._timeout(),
this._timeoutMsg.removeClass("hidden"),
this._loading.addClass("hidden"),
```

while it should look like this:
```
NEWS.CheckIfItemExists.prototype={
init:function(){
this._check()
},
_check:function(){
etc. ........
```

After commenting out just the arrow fuction: **() =>** things go back to normal.

I managed to find another way of doing the setTimeout:

`setTimeout(function () { this._check(); }.bind(this), 1000);`

But obviously the arrow function makes the code simpler and it should be just fine with Cassette.