Update dartdoc to match v4.0.0
Update dartdoc to match v4.0.0

Release notes: https://github.com/dart-lang/dartdoc/releases/tag/v4.0.0

Change-Id: I219ba57cdcd3090b301f40807e5d12256f4e32dc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/215485
Reviewed-by: Janice Collins <f6a8c838e29d3fa8813240c8bed106e760081e0e@google.com>
Reviewed-by: Samuel Rawlins <5cf2cac5eec2ad6d0eb2b0f4ff6af9a590ce2bf6@google.com>
Commit-Queue: Janice Collins <f6a8c838e29d3fa8813240c8bed106e760081e0e@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