Upgrade dartdoc to v0.16.0.

This is for consistency; v0.15.1 and v0.16.0 behave identically when
using the analyzer built in to the SDK.

Change-Id: I50d4bab8eade1cbc9f6537712805fa868af3f573
Reviewed-on: https://dart-review.googlesource.com/40468
Reviewed-by: Devon Carew <devoncarew@google.com>
Commit-Queue: Janice Collins <jcollins@google.com>

DEPS

@@ -83,7 +83,7 @@ vars = {
   # For more details, see https://github.com/dart-lang/sdk/issues/30164
   "dart_style_tag": "@1.0.9",  # Please see the note above before updating.
 
-  "dartdoc_tag" : "@v0.15.1",
+  "dartdoc_tag" : "@v0.16.0",
   "fixnum_tag": "@0.10.5",
   "func_rev": "@25eec48146a58967d75330075ab376b3838b18a8",
   "glob_tag": "@1.1.5",