Restrict `@immutable` checks to instance fields

pkg/analyzer/test/generated/hint_code_test.dart

 // Copyright (c) 2016, 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.test.generated.hint_code_test;
 
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/src/error/codes.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/parser.dart';
@@ skipping 1989 matching lines @@
 @immutable
 class A {
   int x;
 }
 ''');
     await computeAnalysisResult(source);
     assertErrors(source, [HintCode.MUST_BE_IMMUTABLE]);
     verify([source]);
   }
 
+  test_mustBeImmutable_instance() async {
+    Source source = addSource(r'''
+import 'package:meta/meta.dart';
+@immutable
+class A {
+  static int x;
+}
+''');
+    await computeAnalysisResult(source);
+    assertErrors(source, []);
+    verify([source]);
+  }
+
   test_mustBeImmutable_extends() async {
     Source source = addSource(r'''
 import 'package:meta/meta.dart';
 @immutable
 class A {}
 class B extends A {
   int x;
 }
 ''');
     await computeAnalysisResult(source);
@@ skipping 2202 matching lines @@
   n() {
     var a = m(), b = m();
   }
 }''');
     await computeAnalysisResult(source);
     assertErrors(
         source, [HintCode.USE_OF_VOID_RESULT, HintCode.USE_OF_VOID_RESULT]);
     verify([source]);
   }
 }