Update dartdoc to head to help analyzer.

This will facilitate https://dart-review.googlesource.com/c/sdk/+/166680

Change-Id: I4d300df36cb2d088bbd92b231a56b15b2d5fbaf4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/166880
Auto-Submit: Janice Collins <[email protected]>
Commit-Queue: Janice Collins <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: jcollins-g

DEPS

@@ -97,7 +97,7 @@ vars = {
   "dart_style_tag": "1.3.7",  # Please see the note above before updating.
 
   "chromedriver_tag": "83.0.4103.39",
-  "dartdoc_rev" : "v0.35.0",
+  "dartdoc_rev" : "8f5f30e58bbc0f11f104888ee87f11cbd6b82cc7",
   "ffi_rev": "454ab0f9ea6bd06942a983238d8a6818b1357edb",
   "fixnum_rev": "16d3890c6dc82ca629659da1934e412292508bba",
   "glob_rev": "e9f4e6b7ae8abe5071461cf8f47191bb19cf7ef6",