Restrict `@immutable` checks to instance fields

pkg/analyzer/lib/src/dart/error/hint_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.dart.error.hint_codes;
 
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 
 /**
@@ skipping 341 matching lines @@
       "return statement.",
       "Try adding a return statement, or changing the return type to 'void'.");
 
   /**
    * Generate a hint for classes that inherit from classes annotated with
    * `@immutable` but that are not immutable.
    */
   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`
    * that do not invoke the overridden super method.
    *
    * Parameters:
    * 0: the name of the class declaring the overridden method
    */
   static const HintCode MUST_CALL_SUPER = const HintCode(
       'MUST_CALL_SUPER',
@@ skipping 278 matching lines @@
    */
   const HintCode(String name, String message, [String correction])
       : super(name, message, correction);
 
   @override
   ErrorSeverity get errorSeverity => ErrorType.HINT.severity;
 
   @override
   ErrorType get type => ErrorType.HINT;
 }