Restrict `@immutable` checks to instance fields

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

Index: pkg/analyzer/lib/src/dart/error/hint_codes.dart
diff --git a/pkg/analyzer/lib/src/dart/error/hint_codes.dart b/pkg/analyzer/lib/src/dart/error/hint_codes.dart
index 4cfce64ded54abeeab19782e76185be3fc370012..9f19a7f5fb3c8d5cc4e02d85e399811a7a8171d9 100644
--- a/pkg/analyzer/lib/src/dart/error/hint_codes.dart
+++ b/pkg/analyzer/lib/src/dart/error/hint_codes.dart
@@ -359,7 +359,7 @@ class HintCode extends ErrorCode {
   static const HintCode MUST_BE_IMMUTABLE = const HintCode(
       'MUST_BE_IMMUTABLE',
       "This class inherits from a class marked as @immutable, "
-      "and therefore should be immutable (all fields must be final).");
+      "and therefore should be immutable (all instance fields must be final).");
 
   /**
    * Generate a hint for methods that override methods annotated `@mustCallSuper`