Upgrade dartdoc to 0.30.4.
Upgrade dartdoc to 0.30.4.

Release notes:  https://github.com/dart-lang/dartdoc/releases/tag/v0.30.4

Change-Id: Ibce996bd4adb993db9e50c7601d24d4e3fca70da
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/143564
Commit-Queue: Janice Collins <f6a8c838e29d3fa8813240c8bed106e760081e0e@google.com>
Commit-Queue: Samuel Rawlins <5cf2cac5eec2ad6d0eb2b0f4ff6af9a590ce2bf6@google.com>
Auto-Submit: Janice Collins <f6a8c838e29d3fa8813240c8bed106e760081e0e@google.com>
Reviewed-by: Samuel Rawlins <5cf2cac5eec2ad6d0eb2b0f4ff6af9a590ce2bf6@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