Use new strict-casts option in dart_internal

Bug: https://github.com/dart-lang/sdk/issues/47902
Change-Id: Ica869d6884e7ab47c90f893662f759c9e89021de
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/225043
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

pkg/dart_internal/analysis_options.yaml

@@ -1,5 +1,5 @@
 analyzer:
   errors:
     import_internal_library: ignore
-  strong-mode:
-    implicit-casts: false
+  language:
+    strict-casts: true