Update dartdoc to 2.0.0.

Release notes:  https://github.com/dart-lang/dartdoc/releases/tag/v2.0.0

Change-Id: If9a7218a974225224d5f2dd23c026366e71a9405
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/210900
Auto-Submit: Janice Collins <[email protected]>
Commit-Queue: Janice Collins <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>

Author: jcollins-g

DEPS

@@ -105,7 +105,7 @@ vars = {
   # For more details, see https://github.com/dart-lang/sdk/issues/30164
   "dart_style_rev": "14d9b6fd58cc4744676c12be3cc5eee2a779db82",
 
-  "dartdoc_rev" : "5f39ec674d81f5c199151d823fa4ecd01fc59eb2",
+  "dartdoc_rev" : "a4ca86f9bf732d7adc4506f7373e0ed63251b646",
   "devtools_rev" : "2b47d9ed486479153ca2fd038000950674ed1beb",
   "jsshell_tag": "version:88.0",
   "ffi_rev": "4dd32429880a57b64edaf54c9d5af8a9fa9a4ffb",