Migrate from no-implicit-casts to strict-casts (dart-lang/sse#74)

pkgs/sse/analysis_options.yaml

diff --git a/pkgs/sse/analysis_options.yaml b/pkgs/sse/analysis_options.yaml
index 1f65ccf..f370a9d 100644
--- a/pkgs/sse/analysis_options.yaml
+++ b/pkgs/sse/analysis_options.yaml
@@ -1,8 +1,8 @@
 include: package:lints/recommended.yaml
 
 analyzer:
-  strong-mode:
-    implicit-casts: false
+  language:
+    strict-casts: true
   errors:
     dead_code: error
     unused_element: error