Trace reassemble and see if there's any easy wins

[Hixie]

We should trace a reassemble of the Flutter gallery and see why it takes over a second.

cc @johnmccutchan

[Hixie]

I did this. We discussed several things we could do, the most effective of which would be to have the VM notify the instances that it updated somehow, but there's no need to track this here.

[github-actions]

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