support `@virtual` fields, fix #27384

pkg/analyzer/lib/src/dart/element/member.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.element.member;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/constant/value.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
@@ skipping 258 matching lines @@
   ClassElement get enclosingElement => baseElement.enclosingElement;
 
   @override
   PropertyAccessorElement get getter =>
       PropertyAccessorMember.from(baseElement.getter, definingType);
 
   @override
   bool get isEnumConstant => baseElement.isEnumConstant;
 
   @override
+  bool get isVirtual => baseElement.isVirtual;
+
+  @override
   DartType get propagatedType => substituteFor(baseElement.propagatedType);
 
   @override
   PropertyAccessorElement get setter =>
       PropertyAccessorMember.from(baseElement.setter, definingType);
 
   @override
   accept(ElementVisitor visitor) => visitor.visitFieldElement(this);
 
   @override
@@ skipping 772 matching lines @@
   DartObject computeConstantValue() => baseElement.computeConstantValue();
 
   @override
   void visitChildren(ElementVisitor visitor) {
     // TODO(brianwilkerson) We need to finish implementing the accessors used
     // below so that we can safely invoke them.
     super.visitChildren(visitor);
     baseElement.initializer?.accept(visitor);
   }
 }