Support const subclasses of visitor classes (issue 42853)

Change-Id: Idee546d25a4cb96d790d966695c852c22f4c3392
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/156064
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: bwilkerson

pkg/analyzer/lib/dart/ast/visitor.dart

@@ -98,7 +98,7 @@ class DelegatingAstVisitor<T> extends UnifyingAstVisitor<T> {
 
   /// Initialize a newly created visitor to use each of the given delegate
   /// visitors to visit the nodes of an AST structure.
-  DelegatingAstVisitor(this.delegates);
+  const DelegatingAstVisitor(this.delegates);
 
   @override
   T visitNode(AstNode node) {
@@ -128,6 +128,9 @@ class DelegatingAstVisitor<T> extends UnifyingAstVisitor<T> {
 ///
 /// Clients may extend this class.
 class GeneralizingAstVisitor<R> implements AstVisitor<R> {
+  /// Initialize a newly created visitor.
+  const GeneralizingAstVisitor();
+
   @override
   R visitAdjacentStrings(AdjacentStrings node) => visitStringLiteral(node);
 
@@ -606,6 +609,9 @@ class GeneralizingAstVisitor<R> implements AstVisitor<R> {
 ///
 /// Clients may extend this class.
 class RecursiveAstVisitor<R> implements AstVisitor<R> {
+  /// Initialize a newly created visitor.
+  const RecursiveAstVisitor();
+
   @override
   R visitAdjacentStrings(AdjacentStrings node) {
     node.visitChildren(this);
@@ -1341,6 +1347,9 @@ class RecursiveAstVisitor<R> implements AstVisitor<R> {
 ///
 /// Clients may extend this class.
 class SimpleAstVisitor<R> implements AstVisitor<R> {
+  /// Initialize a newly created visitor.
+  const SimpleAstVisitor();
+
   @override
   R visitAdjacentStrings(AdjacentStrings node) => null;
 
@@ -1719,6 +1728,9 @@ class SimpleAstVisitor<R> implements AstVisitor<R> {
 ///
 /// Clients may extend this class.
 class ThrowingAstVisitor<R> implements AstVisitor<R> {
+  /// Initialize a newly created visitor.
+  const ThrowingAstVisitor();
+
   @override
   R visitAdjacentStrings(AdjacentStrings node) => _throw(node);
 
@@ -3097,6 +3109,9 @@ class TimedAstVisitor<T> implements AstVisitor<T> {
 ///
 /// Clients may extend this class.
 class UnifyingAstVisitor<R> implements AstVisitor<R> {
+  /// Initialize a newly created visitor.
+  const UnifyingAstVisitor();
+
   @override
   R visitAdjacentStrings(AdjacentStrings node) => visitNode(node);
 

pkg/analyzer/lib/dart/element/visitor.dart

@@ -79,6 +79,9 @@ import 'package:analyzer/dart/element/element.dart';
 ///
 /// Clients may extend this class.
 class GeneralizingElementVisitor<R> implements ElementVisitor<R> {
+  /// Initialize a newly created visitor.
+  const GeneralizingElementVisitor();
+
   @override
   R visitClassElement(ClassElement element) => visitElement(element);
 
@@ -191,6 +194,9 @@ class GeneralizingElementVisitor<R> implements ElementVisitor<R> {
 ///
 /// Clients may extend this class.
 class RecursiveElementVisitor<R> implements ElementVisitor<R> {
+  /// Initialize a newly created visitor.
+  const RecursiveElementVisitor();
+
   @override
   R visitClassElement(ClassElement element) {
     element.visitChildren(this);
@@ -325,6 +331,9 @@ class RecursiveElementVisitor<R> implements ElementVisitor<R> {
 ///
 /// Clients may extend this class.
 class SimpleElementVisitor<R> implements ElementVisitor<R> {
+  /// Initialize a newly created visitor.
+  const SimpleElementVisitor();
+
   @override
   R visitClassElement(ClassElement element) => null;
 
@@ -398,6 +407,9 @@ class SimpleElementVisitor<R> implements ElementVisitor<R> {
 ///
 /// Clients may extend this class.
 class ThrowingElementVisitor<R> implements ElementVisitor<R> {
+  /// Initialize a newly created visitor.
+  const ThrowingElementVisitor();
+
   @override
   R visitClassElement(ClassElement element) => _throw(element);