Update to dartdoc 0.28.0.

Dependency on mustache4dart replaced with mustache (1.1.0)

Release notes:  https://github.com/dart-lang/dartdoc/releases/tag/v0.28.0

Change-Id: I767b5d4b032e4e0bfe9b5d8e43d61c4c360ff370
Reviewed-on: https://dart-review.googlesource.com/c/89920
Reviewed-by: Devon Carew <devoncarew@google.com>
Commit-Queue: Janice Collins <jcollins@google.com>
diff --git a/.packages b/.packages
index 597ffb9..5426254 100644
--- a/.packages
+++ b/.packages
@@ -63,7 +63,7 @@
 meta:pkg/meta/lib
 mime:third_party/pkg/mime/lib
 mockito:third_party/pkg/mockito/lib
-mustache4dart:third_party/pkg/mustache4dart/lib
+mustache:third_party/pkg/mustache/lib
 oauth2:third_party/pkg/oauth2/lib
 observatory:runtime/observatory/lib
 package_config:third_party/pkg_tested/package_config/lib
diff --git a/DEPS b/DEPS
index c52a98d..501f962 100644
--- a/DEPS
+++ b/DEPS
@@ -81,7 +81,7 @@
   # For more details, see https://github.com/dart-lang/sdk/issues/30164
   "dart_style_tag": "1.2.2",  # Please see the note above before updating.
 
-  "dartdoc_tag" : "v0.27.0",
+  "dartdoc_tag" : "v0.28.0",
   "file_rev": "515ed1dd48740ab14b625de1be464cb2bca4fefd",  # 5.0.6
   "fixnum_tag": "0.10.9",
   "func_rev": "25eec48146a58967d75330075ab376b3838b18a8",
@@ -103,7 +103,7 @@
   "matcher_tag": "0.12.3",
   "mime_tag": "0.9.6+2",
   "mockito_tag": "d39ac507483b9891165e422ec98d9fb480037c8b",
-  "mustache4dart_tag" : "v2.1.2",
+  "mustache_tag" : "5e81b12215566dbe2473b2afd01a8a8aedd56ad9",
   "oauth2_tag": "1.2.1",
   "observatory_pub_packages_rev": "0894122173b0f98eb08863a7712e78407d4477bc",
   "package_config_tag": "1.0.5",
@@ -281,10 +281,10 @@
       Var("dart_git") + "mime.git" + "@" + Var("mime_tag"),
   Var("dart_root") + "/third_party/pkg/mockito":
       Var("dart_git") + "mockito.git" + "@" + Var("mockito_tag"),
-  Var("dart_root") + "/third_party/pkg/mustache4dart":
-      Var("chromium_git")
-      + "/external/github.com/valotas/mustache4dart.git"
-      + "@" + Var("mustache4dart_tag"),
+  Var("dart_root") + "/third_party/pkg/mustache":
+      Var("dart_git")
+      + "external/github.com/xxgreg/mustache"
+      + "@" + Var("mustache_tag"),
   Var("dart_root") + "/third_party/pkg/oauth2":
       Var("dart_git") + "oauth2.git" + "@" + Var("oauth2_tag"),
   Var("dart_root") + "/third_party/observatory_pub_packages":