Migrate from no-implicit-casts to strict-casts (dart-lang/markdown#504)

pkgs/markdown/analysis_options.yaml

diff --git a/pkgs/markdown/analysis_options.yaml b/pkgs/markdown/analysis_options.yaml
index ac272aa..b6a1426 100644
--- a/pkgs/markdown/analysis_options.yaml
+++ b/pkgs/markdown/analysis_options.yaml
@@ -3,10 +3,9 @@
 
 analyzer:
   language:
+    strict-casts: true
     strict-inference: true
     strict-raw-types: true
-  strong-mode:
-    implicit-casts: false
   errors:
     dead_code: error
     unused_element: error