Update to latest package:csslib

Removes a cast that was made unnecessary with a fix to flow analysis.

Change-Id: Ia80493fbe64abddd268ca468a7860044954a3928
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/165143
Auto-Submit: Nate Bosch <nbosch@google.com>
Commit-Queue: Nate Bosch <nbosch@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>
diff --git a/DEPS b/DEPS
index d86ab2b..87d2db4 100644
--- a/DEPS
+++ b/DEPS
@@ -79,7 +79,7 @@
   "collection_rev": "52e219581f72a3eac013d6f5550c580962677425",
   "convert_rev": "c1b01f832835d3d8a06b0b246a361c0eaab35d3c",
   "crypto_rev": "f7c48b334b1386bc5ab0f706fbcd6df8496a87fc",
-  "csslib_rev": "681a6603f86bbbe64e8af42d722aef63b028d241",
+  "csslib_rev": "6f77b3dcee957d3e2d5083f666221a220e9ed1f1",
   "dart2js_info_rev" : "0632a623b08e1f601c7eba99e0186a581ae799e9",
 
   # Note: Updates to dart_style have to be coordinated with the infrastructure