Update dartdoc to 0.38.0.
Update dartdoc to 0.38.0.

Release notes:  https://github.com/dart-lang/dartdoc/releases/tag/v0.38.0


Change-Id: Idac8b626e279df5fae5f8837a5327a2c7b54395d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/175381
Reviewed-by: Devon Carew <e996497f4f2225f0ee7a9f4d1696ee710b488b66@google.com>
Reviewed-by: Samuel Rawlins <5cf2cac5eec2ad6d0eb2b0f4ff6af9a590ce2bf6@google.com>
Commit-Queue: Devon Carew <e996497f4f2225f0ee7a9f4d1696ee710b488b66@google.com>
Auto-Submit: 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