What problem does this feature solve?

This would allow the router to read navigation guards added through mixins without having to instantiate the components. It was possible in Vue 2 with Vue.extend

From https://github.com/vuejs/vue-router-next/issues/454

What does the proposed API look like?

resolveComponentOptions(componentOptions: ComponentOptions, app?: App): ComponentOptions

• apply local mixins option
• if app is provided, also apply global mixins

This is of course just an idea, maybe it doesn't make sense to allow passing app.

It could also be a function like applyMixins(componentOptions) and I could append app._context.mixins