async components in slots don't render on the first pass

[719media]

Version

2.6.2

Reproduction link

https://codesandbox.io/s/p530yln9z7

Steps to reproduce

Open the repro, follow the instructions.

What is expected?

Component b should appear

What is actually happening?

Component b does not appear

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Using Vue 2.5.22 works, or not async loading the component works as well

[719media]

Removing the slot-scope="props" also fixes the problem. Granted, there are no v-bind on the slot definition, so it's a little funny that slot-scope is even used in the repro, but this is a minimal repro so I hoped it would still show the problem to you. I can attempt to come up with a better repro if needed.