We try to be compatible with Google Closure Compiler advanced mode, which crushes object keys. (So `spec.mixins` becomes `spec.zb` or something like that.) In order to make sure that the key we're checking here is the same as the crushed version, you should do

```
var MIXINS_KEY = keyOf({mixins: null});
```

at the top of the file and then use MIXINS_KEY here instead of `'mixins'` directly.