implicit-casts:false on flutter_tools (#45153)

* implicit-casts:false on flutter_tools

* use castStringKeyedMap

* address review comments

* address review comments

* fix issues after rebase
diff --git a/packages/flutter_tools/test/commands.shard/hermetic/version_test.dart b/packages/flutter_tools/test/commands.shard/hermetic/version_test.dart
index f58c8c9..45a7025 100644
--- a/packages/flutter_tools/test/commands.shard/hermetic/version_test.dart
+++ b/packages/flutter_tools/test/commands.shard/hermetic/version_test.dart
@@ -117,7 +117,7 @@
       return ProcessResult(0, 0, 'v10.0.0\r\nv20.0.0', '');
     }
     if (command[0] == 'git' && command[1] == 'checkout') {
-      version = command[2];
+      version = command[2] as String;
     }
     return ProcessResult(0, 0, '', '');
   }