Turn the DOWN_CAST_COMPOSITE strong mode error into a hint, which will

pkg/analyzer/lib/src/error/codes.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.error.codes;
 
 import 'package:analyzer/error/error.dart';
 
 export 'package:analyzer/src/analysis_options/error/option_codes.dart';
 export 'package:analyzer/src/dart/error/hint_codes.dart';
@@ skipping 4701 matching lines @@
    * result of turning on a particular option, and they are free to turn it
    * back off.
    */
   static const String _implicitDynamicCorrection =
       "Try adding an explicit type like 'dynamic', or "
       "enable implicit-dynamic in your analysis options file.";
 
   static const String _inferredTypeMessage = "'{0}' has inferred type '{1}'.";
 
   static const StrongModeCode DOWN_CAST_COMPOSITE = const StrongModeCode(
-      ErrorType.STATIC_WARNING,
+      ErrorType.HINT,
       'DOWN_CAST_COMPOSITE',
       _implicitCastMessage,
       _implicitCastCorrection);
 
   static const StrongModeCode DOWN_CAST_IMPLICIT = const StrongModeCode(
       ErrorType.HINT,
       'DOWN_CAST_IMPLICIT',
       _implicitCastMessage,
       _implicitCastCorrection);
 
@@ skipping 210 matching lines @@
    * created from the optional [correction] template.
    */
   const StrongModeCode(ErrorType type, String name, String message,
       [String correction])
       : type = type,
         super('STRONG_MODE_$name', message, correction);
 
   @override
   ErrorSeverity get errorSeverity => type.severity;
 }