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

hide it unless you use an analysis options file to promote it to a warning
or an error.

R=vsm@google.com
BUG=

Review-Url: https://codereview.chromium.org/2734343002 .

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

@@ -4733,7 +4733,7 @@ class StrongModeCode extends ErrorCode {
   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);