analyzer_plugin: switch to use strict-casts

Bug: https://github.com/dart-lang/sdk/issues/41651
Change-Id: I3f987e6ed11b8f53ab460227a6ffc02b6fbb57f4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/222383
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

pkg/analyzer_plugin/analysis_options.yaml

@@ -1,6 +1,6 @@
 analyzer:
-  strong-mode:
-    implicit-casts: false
+  language:
+    strict-casts: true
 linter:
   rules:
     - await_only_futures