Roll Dart Documentation Generator from adc5a45ddafb to 621c9192cc24 (4 revisions)

https://dart.googlesource.com/dartdoc/+log/adc5a45ddafb..621c9192cc24

2022-07-12 srawlins@google.com Move annotation tests out of end-to-end test (#3078)
2022-07-11 srawlins@google.com Move test skip above test body (#3076)
2022-07-11 srawlins@google.com Remove ElementType.returnedFrom; was completely unused (#3077)
2022-07-09 srawlins@google.com Make Mixin.superclassConstraints late final non-nullable (#3075)

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/dart-doc-dart-sdk
Please CC dart-ecosystem-gardener@grotations.appspotmail.com on the revert to ensure that a human
is aware of the problem.

To file a bug in Dart Documentation Generator: https://github.com/dart-lang/dartdoc/issues
To file a bug in Dart SDK: https://github.com/dart-lang/sdk/issues

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

Tbr: dart-ecosystem-gardener@grotations.appspotmail.com
Change-Id: I82e92bf6c915969b2ba72de470939673f63b4de9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/251161
Commit-Queue: Nate Bosch <nbosch@google.com>
Reviewed-by: Nate Bosch <nbosch@google.com>

DEPS

@@ -108,7 +108,7 @@ vars = {
   # For more details, see https://github.com/dart-lang/sdk/issues/30164.
   "dart_style_rev": "d7b73536a8079331c888b7da539b80e6825270ea", # manually rev'd
 
-  "dartdoc_rev": "adc5a45ddafb57e3a600ee7d9e88dc81061d1410",
+  "dartdoc_rev": "621c9192cc2446ff1ebe4f603e1c834392d8b547",
   "devtools_rev": "95d292626da26505b02417735f77e8922783b477",
   "ffi_rev": "18b2b549d55009ff594600b04705ff6161681e07",
   "file_rev": "0132eeedea2933513bf230513a766a8baeab0c4f",