returned from `defineComponent`
ref: https://github.com/vuejs/vue-router-next/pull/421
also close#1880
Previous `Component` type is now exported as `ConcreteComponent`.
This introduces a minor breaking change when calling `h(comp, { ... })`
will now fail if `comp` is a of generic `Component` type, since it does
not specify what props it expects.
BREAKING CHANGE: `retryWhen` and `maxRetries` options for
`defineAsyncComponent` has been replaced by the more flexible `onError`
option, per https://github.com/vuejs/rfcs/pull/148
