Update dartdoc to 0.40.0.

Release notes:  https://github.com/dart-lang/dartdoc/releases/tag/v0.40.0

Change-Id: I9382db0e980cc4cefa4f0a647acba09c945b756a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/187680
Auto-Submit: Janice Collins <jcollins@google.com>
Commit-Queue: Janice Collins <jcollins@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Devon Carew <devoncarew@google.com>
diff --git a/.dart_tool/package_config.json b/.dart_tool/package_config.json
index a203927..45febc4 100644
--- a/.dart_tool/package_config.json
+++ b/.dart_tool/package_config.json
@@ -11,7 +11,7 @@
     "constraint, update this by running tools/generate_package_config.dart."
   ],
   "configVersion": 2,
-  "generated": "2021-02-25T12:53:17.111860",
+  "generated": "2021-03-01T09:26:17.118461",
   "generator": "tools/generate_package_config.dart",
   "packages": [
     {
diff --git a/DEPS b/DEPS
index 35ba597..383b118 100644
--- a/DEPS
+++ b/DEPS
@@ -101,7 +101,7 @@
   "dart_style_tag": "1.3.11",  # Please see the note above before updating.
 
   "chromedriver_tag": "83.0.4103.39",
-  "dartdoc_rev" : "9e61a4f11091aaa8998525a2692b14148dc24ab5",
+  "dartdoc_rev" : "174021f3d76c2a8500b54977c80f32c5bbc099f1",
   "ffi_rev": "f3346299c55669cc0db48afae85b8110088bf8da",
   "fixnum_rev": "16d3890c6dc82ca629659da1934e412292508bba",
   "file_rev": "0e09370f581ab6388d46fda4cdab66638c0171a1",