support `@virtual` fields, fix #27384

pkg/analyzer/test/src/task/strong/checker_test.dart

 // Copyright (c) 2015, 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.src.task.strong.checker_test;
 
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import 'strong_test_helper.dart';
 
@@ skipping 522 matching lines @@
   }
 
   void test_covariantOverride_fields() {
     _addMetaLibrary();
     checkFile(r'''
 import 'meta.dart';
 class A {
   get foo => '';
   set foo(_) {}
 }
+
 class B extends A {
-  @checked int foo;
+  @checked num foo;
+}
+class C extends A {
+  @checked @virtual num foo;
+}
+class D extends C {
+  @virtual int foo;
+}
+class E extends D {
+  @virtual /*error:INVALID_METHOD_OVERRIDE*/num foo;
 }
     ''');
   }
 
   void test_covariantOverride_leastUpperBound() {
     _addMetaLibrary();
     checkFile(r'''
 import "meta.dart";
 abstract class Top {}
 abstract class Left implements Top {}
@@ skipping 190 matching lines @@
   /*error:INVALID_FIELD_OVERRIDE, error:INVALID_METHOD_OVERRIDE*/final ToVoid<dynamic> g = null;
 }
 
 class H implements F {
   final ToVoid<int> f = null;
   /*error:INVALID_METHOD_OVERRIDE*/final ToVoid<dynamic> g = null;
 }
  ''');
   }
 
+  void test_fieldOverride_virtual() {
+    _addMetaLibrary();
+    checkFile(r'''
+import 'meta.dart';
+class C {
+  @virtual int x;
+}
+class OverrideGetter extends C {
+  int get x => 42;
+}
+class OverrideSetter extends C {
+  set x(int v) {}
+}
+class OverrideBoth extends C {
+  int get x => 42;
+  set x(int v) {}
+}
+class OverrideWithField extends C {
+  int x;
+
+  // expose the hidden storage slot
+  int get superX => super.x;
+  set superX(int v) { super.x = v; }
+}
+class VirtualNotInherited extends OverrideWithField {
+  /*error:INVALID_FIELD_OVERRIDE*/int x;
+}
+    ''');
+  }
+
   void test_fieldSetterOverride() {
     checkFile('''
 class A {}
 class B extends A {}
 class C extends B {}
 
 class Base {
   B f1;
   B f2;
   B f3;
@@ skipping 3104 matching lines @@
 }
 ''');
   }
 }
 
 void _addMetaLibrary() {
   addFile(r'''
 library meta;
 class _Checked { const _Checked(); }
 const Object checked = const _Checked();
+
+class _Virtual { const _Virtual(); }
+const Object virtual = const _Virtual();
     ''', name: '/meta.dart');
 }