Remove strong_mode_down_cast_composite from .analysis_options.

pkg/front_end/.analysis_options

Index: pkg/front_end/.analysis_options
diff --git a/pkg/front_end/.analysis_options b/pkg/front_end/.analysis_options
index 61936d8ce50b96d5f5e320b37789a19ebca40b02..07aef1375c237fca56b9554d92648235a2c447c9 100644
--- a/pkg/front_end/.analysis_options
+++ b/pkg/front_end/.analysis_options
@@ -10,11 +10,6 @@ analyzer:
     # TODO(sigmund): enable once #28601 is fixed
     strong_mode_invalid_field_override: ignore
 
-    # Omits warnings for implicit casts with generics like `List<T> x = list;`,
-    # also removes warnings due to a bad interaction between inference of
-    # guarded type arguments and downward cast warnings (see #28646).
-    strong_mode_down_cast_composite: ignore
-
     # Allow having TODOs in the code
     todo: ignore