fix(slots): filter out compiler marker from resolved slots

[underfin]

fix #1470

Bug Reason

It caused by #1451. The pre fix filter out the '_' marker caused the patchflags to PatchFlags.BAIL (fragment withBaseComponent) -> the pre dynamicChildren is null.

slots._ ? PatchFlags.STABLE_FRAGMENT : PatchFlags.BAIL

if (n2.patchFlag === PatchFlags.BAIL) {
      optimized = false
      n2.dynamicChildren = null
    }

[yyx990803]

Good catch. Although it's too expensive to call Object.defineProperty like this. We can simply make the initial slot marker non-enumberable. See 062835d

[underfin]

There places maybe should prevent assign new '_' enumberable flag because it is readonly.
https://github.com/vuejs/vue-next/blob/062835d45aaf4168ddf2e39a5c7e162b3a18ccae/packages/runtime-core/src/componentSlots.ts#L133-L145

[underfin]

=.= I'm curious about why use _ flag make marker compilered slots.Why not add a new static_slots patchFlags instead of it(we has already dynamic_slots flag and them can represent all compiled case)?

The compiled code is clear and we don't nedd process edge case.

[yyx990803]

Because _ already exists and is writable, the new defined property will also be writable.

The reason we don't use a vnode flag is because the slots object may get passed down in a render function. If we only check the vnode's flag, then it is treated as manual/dynamic once passed down. But if we keep the information on the slots object itself, then the optimization can be kept no matter where it is passed to.

[underfin]

Thanks a lot for your explain.