update DEPS to pull in dartdoc version 0.9.7+6.

BUG=
R=devoncarew@google.com

Review URL: https://codereview.chromium.org/2406563002 .
diff --git a/DEPS b/DEPS
index da4429b..83b2770 100644
--- a/DEPS
+++ b/DEPS
@@ -57,7 +57,7 @@
   "dart2js_info_tag" : "@0.2.7+1",
   "dart_services_rev" : "@7aea2574e6f3924bf409a80afb8ad52aa2be4f97",
   "dart_style_tag": "@0.2.10",
-  "dartdoc_tag" : "@v0.9.7+3",
+  "dartdoc_tag" : "@v0.9.7+6",
   "fixnum_tag": "@0.10.5",
   "func_tag": "@0.1.0",
   "glob_tag": "@1.1.3",