Migrate no-implicit-casts to strict-casts (#32)

analysis_options.yaml

diff --git a/analysis_options.yaml b/analysis_options.yaml
index d5e9af7..08787b5 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -1,7 +1,6 @@
 include: package:lints/recommended.yaml
 
 analyzer:
-  strong-mode:
-    implicit-casts: false
   language:
+    strict-casts: true
     strict-raw-types: true