Alert blocks do not render on pub.dev generated docs

[JaffaKetchup]

Not sure if this is the best place for the issue, please transfer if necessary.

When generating locally, the markdown alert blocks appear correctly:

However, in the generated documentation for a pub.dev package, the blocks do not render:

[parlough]

\cc @isoos I know pub.dev manually replicates some of dartdoc's styles, is this just something missing from that?

[isoos]

We do support them, so this seems to be a bug. Was there any change in the output format or styles that dardoc is using? We've just released a new site version with dartdoc 8.0.10...

[isoos]

I've found the culprit: pub.dev executes dartdoc with the --sanitize-html flag, and when that is on, the sanitization removes the alert block's div and related styles.

[srawlins]

Fixed by #3820