Elements. Add ElementVisitor2.

Change-Id: I028a37d4ed1f1df408950cb665aa3c7d3d191978
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/389593
Reviewed-by: Phil Quitslund <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analyzer/lib/dart/element/element2.dart

@@ -497,6 +497,12 @@ abstract class Element2 {
   /// The analysis session in which this element is defined.
   AnalysisSession? get session;
 
+  /// Uses the given [visitor] to visit this element.
+  ///
+  /// Returns the value returned by the visitor as a result of visiting this
+  /// element.
+  T? accept2<T>(ElementVisitor2<T> visitor);
+
   /// The presentation of this element as it should appear when presented to
   /// users.
   ///
@@ -535,6 +541,71 @@ abstract class Element2 {
   ///
   /// Returns `null` if there is no such element.
   E? thisOrAncestorOfType2<E extends Element2>();
+
+  /// Uses the given [visitor] to visit all of the children of this element.
+  /// There is no guarantee of the order in which the children will be visited.
+  void visitChildren2<T>(ElementVisitor2<T> visitor);
+}
+
+/// An object that can be used to visit an element structure.
+///
+/// Clients may not extend, implement or mix-in this class. There are classes
+/// that implement this interface that provide useful default behaviors in
+/// `package:analyzer/dart/element/visitor.dart`. A couple of the most useful
+/// include
+/// * SimpleElementVisitor which implements every visit method by doing nothing,
+/// * RecursiveElementVisitor which will cause every node in a structure to be
+///   visited, and
+/// * ThrowingElementVisitor which implements every visit method by throwing an
+///   exception.
+abstract class ElementVisitor2<R> {
+  R? visitClassElement(ClassElement2 element);
+
+  R? visitConstructorElement(ConstructorElement2 element);
+
+  R? visitEnumElement(EnumElement2 element);
+
+  R? visitExtensionElement(ExtensionElement2 element);
+
+  R? visitExtensionTypeElement(ExtensionTypeElement2 element);
+
+  R? visitFieldElement(FieldElement2 element);
+
+  R? visitFieldFormalParameterElement(FieldFormalParameterElement2 element);
+
+  R? visitFormalParameterElement(FormalParameterElement element);
+
+  R? visitGenericFunctionTypeElement(GenericFunctionTypeElement2 element);
+
+  R? visitGetterElement(GetterElement element);
+
+  R? visitLabelElement(LabelElement2 element);
+
+  R? visitLibraryElement(LibraryElement2 element);
+
+  R? visitLocalFunctionElement(LocalFunctionElement element);
+
+  R? visitLocalVariableElement(LocalVariableElement2 element);
+
+  R? visitMethodElement(MethodElement2 element);
+
+  R? visitMixinElement(MixinElement2 element);
+
+  R? visitMultiplyDefinedElement(MultiplyDefinedElement2 element);
+
+  R? visitPrefixElement(PrefixElement2 element);
+
+  R? visitSetterElement(SetterElement element);
+
+  R? visitSuperFormalParameterElement(SuperFormalParameterElement2 element);
+
+  R? visitTopLevelFunctionElement(TopLevelFunctionElement element);
+
+  R? visitTopLevelVariableElement(TopLevelVariableElement2 element);
+
+  R? visitTypeAliasElement(TypeAliasElement2 element);
+
+  R? visitTypeParameterElement(TypeParameterElement2 element);
 }
 
 /// An element that represents an enum.