esify should not mangle class properties in shorthand hashes


(see `kpi/base.coffee for a real example`)

```coffee
class Foo
  bar: ->
    template = _.template('...')
    template({@prop1, @prop2})
```

The `bar` method is esified to:
```js
bar() {
    const template = _.template('lol');

    return template({
      this: this,
      this: this,
    });
}
```
