Upgrade dartdoc to v0.30.1.
Upgrade dartdoc to v0.30.1.

Change-Id: I8e6b6bad839912ba30c9d03f7763bd89cbadab86
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/136281
Commit-Queue: Janice Collins <f6a8c838e29d3fa8813240c8bed106e760081e0e@google.com>
Commit-Queue: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>
Auto-Submit: Janice Collins <f6a8c838e29d3fa8813240c8bed106e760081e0e@google.com>
Reviewed-by: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk