Reland "Update dartdoc to 0.43.0."

This is a reland of 076341b89aad0299afeee491b77d76657bb2056d

Original change's description:
> Update dartdoc to 0.43.0.
>
> Release notes:  https://github.com/dart-lang/dartdoc/releases/tag/v0.43.0
>
> Change-Id: If60b856ef36a629d89534d5cc9a828b0782937ec
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/199501
> Reviewed-by: Samuel Rawlins <srawlins@google.com>
> Commit-Queue: Janice Collins <jcollins@google.com>

Change-Id: If5746369b2a1fdc05a6a86ff80cacc279cf802d1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/199701
Reviewed-by: Devon Carew <devoncarew@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Janice Collins <jcollins@google.com>
diff --git a/DEPS b/DEPS
index 835c510..f0a9838 100644
--- a/DEPS
+++ b/DEPS
@@ -106,7 +106,7 @@
   "dart_style_rev": "f17c23e0eea9a870601c19d904e2a9c1a7c81470",
 
   "chromedriver_tag": "83.0.4103.39",
-  "dartdoc_rev" : "505f163f7cb48e917503e4a23fbff1227e08b263",
+  "dartdoc_rev" : "e6a9b7c536a85e49233c97bb892bbb0ab778e425",
   "devtools_rev" : "12ad5341ae0a275042c84a4e7be9a6c98db65612",
   "jsshell_tag": "version:88.0",
   "ffi_rev": "f3346299c55669cc0db48afae85b8110088bf8da",