support `@virtual` fields, fix #27384

pkg/analyzer/lib/dart/element/element.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.
 
 /**
  * Defines the element model. The element model describes the semantic (as
  * opposed to syntactic) structure of Dart code. The syntactic structure of the
  * code is modeled by the [AST structure](../ast/ast.dart).
  *
  * The element model consists of two closely related kinds of objects: elements
@@ skipping 1148 matching lines @@
   /**
    * An empty list of field elements.
    */
   static const List<FieldElement> EMPTY_LIST = const <FieldElement>[];
 
   /**
    * Return {@code true} if this element is an enum constant.
    */
   bool get isEnumConstant;
 
+  /**
+   * Returns `true` if this field can be overridden in strong mode.
+   */
+  bool get isVirtual;
+
   @override
   AstNode computeNode();
 }
 
 /**
  * A field formal parameter defined within a constructor element.
  *
  * Clients may not extend, implement or mix-in this class.
  */
 abstract class FieldFormalParameterElement implements ParameterElement {
@@ skipping 827 matching lines @@
   DartType get type;
 
   /**
    * Return a representation of the value of this variable, forcing the value
    * to be computed if it had not previously been computed, or `null` if either
    * this variable was not declared with the 'const' modifier or if the value of
    * this variable could not be computed because of errors.
    */
   DartObject computeConstantValue();
 }