[CFE] Fix dartdoc tests not working on parts

Fixes #61438.

Change-Id: Iea44e80be2b62c17caa4e9830a30f9a0d61bed67
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/448300
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Jens Johansen <[email protected]>

Author: jensjoha