fix #5760: transition async component error

[gebilaoxiong]

issue: #5760

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• Bugfix
• Feature
• Code style update
• Refactor
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• Yes
• No

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• New/updated tests are included

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Other information:

[gebilaoxiong]

It 's too late today and going to work tomorrow

I will fill the unit test in tomorrow ...

sorry

[Kingwl]

IMO 'div' is not a good idea

[gebilaoxiong]

But it is invisible, just a placeholder, used to replace comment

[Kingwl]

ping @yyx990803

[gebilaoxiong]

@Kingwl

Now, I use a transition placeholder...

can you help me to review it, thx : )

[Kingwl]

LGTM

[yyx990803]

Thanks @gebilaoxiong for the hard work. I found out a simpler fix in c3cdfcf but your PR helped a lot in identifying the problem.

[gebilaoxiong]

@yyx990803

Thanks, your code is pretty

[gebilaoxiong]

@yyx990803 there is a small problem

c3cdfcf#diff-6964744ffd2d3536df0f526fb0e2c286R142

child.key = child.key == null
  ? id + child.tag
  : isPrimitive(child.key)
    ? (String(child.key).indexOf(id) === 0 ? child.key : id + child.key)
    : child.key

if child is async placeholder(comment) and it did not have the key

then the child key is __transition-1-undefined