Upgrade dartdoc to v0.21.1

Changes:

https://github.com/dart-lang/dartdoc/releases/tag/v0.21.1
https://github.com/dart-lang/dartdoc/releases/tag/v0.21.0

Change-Id: Ib6a113b919b8d9718d312d4dc6b7a611593550c2
Reviewed-on: https://dart-review.googlesource.com/75984
Reviewed-by: Devon Carew <devoncarew@google.com>
Commit-Queue: Janice Collins <jcollins@google.com>
diff --git a/DEPS b/DEPS
index 82d3de2..efd5aea 100644
--- a/DEPS
+++ b/DEPS
@@ -80,7 +80,7 @@
   # For more details, see https://github.com/dart-lang/sdk/issues/30164
   "dart_style_tag": "1.2.0",  # Please see the note above before updating.
 
-  "dartdoc_tag" : "v0.20.4",
+  "dartdoc_tag" : "v0.21.1",
   "fixnum_tag": "0.10.8",
   "func_rev": "25eec48146a58967d75330075ab376b3838b18a8",
   "glob_tag": "1.1.7",