Restrict `@immutable` checks to instance fields

pkg/analyzer/test/generated/hint_code_test.dart

Index: pkg/analyzer/test/generated/hint_code_test.dart
diff --git a/pkg/analyzer/test/generated/hint_code_test.dart b/pkg/analyzer/test/generated/hint_code_test.dart
index 93ea7b340d16dbd44fe3fcd0bff55732f03d3b70..f0852ba7191d6869b03c49dad8c2b1e3f635adb0 100644
--- a/pkg/analyzer/test/generated/hint_code_test.dart
+++ b/pkg/analyzer/test/generated/hint_code_test.dart
@@ -2007,6 +2007,19 @@ class A {
     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';