support `@virtual` fields, fix #27384

pkg/analyzer/lib/src/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.
 
 library analyzer.src.dart.element.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 2496 matching lines @@
    */
   static String _REQUIRED_CLASS_NAME = "Required";
 
   /**
    * The name of the top-level variable used to mark a parameter as being
    * required.
    */
   static String _REQUIRED_VARIABLE_NAME = "required";
 
   /**
+   * The name of the top-level variable used to mark a member as intended to be
+   * overridden.
+   */
+  static String _VIRTUAL_VARIABLE_NAME = "virtual";
+
+  /**
    * The element representing the field, variable, or constructor being used as
    * an annotation.
    */
   Element element;
 
   /**
    * The compilation unit in which this annotation appears.
    */
   CompilationUnitElementImpl compilationUnit;
 
@@ skipping 82 matching lines @@
   @override
   bool get isRequired =>
       element is ConstructorElement &&
           element.enclosingElement.name == _REQUIRED_CLASS_NAME &&
           element.library?.name == _META_LIB_NAME ||
       element is PropertyAccessorElement &&
           element.name == _REQUIRED_VARIABLE_NAME &&
           element.library?.name == _META_LIB_NAME;
 
   /**
+   * Return `true` if this annotation marks the associated member as supporting
+   * overrides.
+   *
+   * This is currently used by fields in Strong Mode, as other members are
+   * already virtual-by-default.
+   */
+  bool get isVirtual =>
+      element is PropertyAccessorElement &&
+      element.name == _VIRTUAL_VARIABLE_NAME &&
+      element.library?.name == _META_LIB_NAME;
+
+  /**
    * Get the library containing this annotation.
    */
   Source get librarySource => compilationUnit.librarySource;
 
   @override
   Source get source => compilationUnit.source;
 
   @override
   DartObject computeConstantValue() {
     if (evaluationResult == null) {
@@ skipping 1546 matching lines @@
 
   @override
   bool get isStatic {
     if (_unlinkedVariable != null) {
       return _unlinkedVariable.isStatic;
     }
     return hasModifier(Modifier.STATIC);
   }
 
   @override
+  bool get isVirtual {
+    for (ElementAnnotationImpl annotation in metadata) {
+      if (annotation.isVirtual) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  @override
   ElementKind get kind => ElementKind.FIELD;
 
   /**
    * Set whether this field is static.
    */
   void set static(bool isStatic) {
     assert(_unlinkedVariable == null);
     setModifier(Modifier.STATIC, isStatic);
   }
 
@@ skipping 4169 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }