Make devtools usable once more.

[jdm]

This fixes the panic by rebasing #6189, and also makes cached messages appear once more.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/5455

This is an external review system which you may optionally use for the code review of your pull request.

In order to help critic track your changes, please do not make in-place history rewrites (e.g. via git rebase -i or git commit --amend) when updating this pull request.

[glennw]

Reviewed 1 of 1 files at r1, 3 of 3 files at r2.
Review status: all files reviewed at latest revision, 2 unresolved discussions, all commit checks successful.

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^{components/devtools/lib.rs, line 255 [r2] (raw file):}
Should we print a debug warning here or is it expected?

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^{components/devtools/lib.rs, line 306 [r2] (raw file):}
Should we print a debug warning here or is it expected?

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Comments from the review on Reviewable.io

[jdm]

Review status: all files reviewed at latest revision, 2 unresolved discussions, all commit checks successful.

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^{components/devtools/lib.rs, line 255 [r2] (raw file):}
Expected.

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^{components/devtools/lib.rs, line 306 [r2] (raw file):}
Expected.

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Comments from the review on Reviewable.io

[glennw]

@bors-servo r+

[bors-servo]

📌 Commit 81c0d32 has been approved by glennw

[bors-servo]

⌛ Testing commit 81c0d32 with merge c04b7bb...

[bors-servo]

☀️ Test successful - android, gonk, linux1, linux2, linux3, mac1, mac2, mac3