Migrate from no-implicit-casts to strict-casts (dart-lang/watcher#134)

pkgs/watcher/analysis_options.yaml

diff --git a/pkgs/watcher/analysis_options.yaml b/pkgs/watcher/analysis_options.yaml
index 29327a4..4320162 100644
--- a/pkgs/watcher/analysis_options.yaml
+++ b/pkgs/watcher/analysis_options.yaml
@@ -1,8 +1,8 @@
 include: package:lints/recommended.yaml
 
 analyzer:
-  strong-mode:
-    implicit-casts: false
+  language:
+    strict-casts: true
   errors:
     todo: ignore
     unused_import: error