Migrate from no-implicit-casts to strict-casts (dart-lang/stream_channel#84)

pkgs/stream_channel/analysis_options.yaml

diff --git a/pkgs/stream_channel/analysis_options.yaml b/pkgs/stream_channel/analysis_options.yaml
index 8465834..517b597 100644
--- a/pkgs/stream_channel/analysis_options.yaml
+++ b/pkgs/stream_channel/analysis_options.yaml
@@ -1,8 +1,8 @@
 include: package:lints/recommended.yaml
 
 analyzer:
-  strong-mode:
-    implicit-casts: false
+  language:
+    strict-casts: true
   # These are errors when building in Google
   errors:
     unused_import: error