lone mixin in module not present in output custom-elements.json

[thepassle]

if I have a mixin in a module like so:

export const MyMixin = klass => class MyMixin extends klass {}

I'd expect a:

        {
          "kind": "variable",
          "name": "MyMixin",
          "type": "(klass: any) => typeof MyMixin"
        }

in the output custom-elements.json, but this is not the case. This only works if I include the mixin in the same file as a custom element. But in real projects, its more likely that Mixins are in their own module, and are imported.