refactor(normalize,ferry,ferry_generator): Add const constructor to visitors

docs/customization.md

@@ -69,7 +69,7 @@ class AddTypenameTypedLink extends TypedLink {
           ..operation = Operation(
             document: transform(
               request.operation.document,
-              [AddTypenameVisitor()],
+              const [AddTypenameVisitor()],
             ),
             operationName: request.operation.operationName,
           ),

packages/ferry/lib/src/add_typename_typed_link.dart

@@ -22,7 +22,7 @@ class AddTypenameTypedLink extends TypedLink {
         (op) => Operation(
           document: transform(
             op.document,
-            [
+            const [
               AddTypenameVisitor(),
             ],
           ),

packages/ferry_generator/lib/src/utils/add_introspection.dart

@@ -14,6 +14,8 @@ const typenameField = FieldDefinitionNode(
 );
 
 class AddTypenameField extends TransformingVisitor {
+  const AddTypenameField();
+
   @override
   ObjectTypeDefinitionNode visitObjectTypeDefinitionNode(
     ObjectTypeDefinitionNode node,

packages/normalize/lib/src/denormalize_fragment.dart

@@ -40,7 +40,7 @@ Map<String, dynamic>? denormalizeFragment({
   if (addTypename) {
     document = transform(
       document,
-      [AddTypenameVisitor()],
+      const [AddTypenameVisitor()],
     );
   }
   final fragmentMap = getFragmentMap(document);

packages/normalize/lib/src/denormalize_operation.dart

@@ -35,7 +35,7 @@ Map<String, dynamic>? denormalizeOperation({
   if (addTypename) {
     document = transform(
       document,
-      [AddTypenameVisitor()],
+      const [AddTypenameVisitor()],
     );
   }
 

packages/normalize/lib/src/normalize_fragment.dart

@@ -44,7 +44,7 @@ void normalizeFragment({
   // Always add typenames to ensure data is stored with typename
   document = transform(
     document,
-    [AddTypenameVisitor()],
+    const [AddTypenameVisitor()],
   );
 
   final fragmentMap = getFragmentMap(document);

packages/normalize/lib/src/normalize_operation.dart

@@ -40,7 +40,7 @@ void normalizeOperation({
   if (addTypename) {
     document = transform(
       document,
-      [AddTypenameVisitor()],
+      const [AddTypenameVisitor()],
     );
   }
 

packages/normalize/lib/src/utils/add_typename_visitor.dart

@@ -1,6 +1,8 @@
 import 'package:gql/ast.dart';
 
 class AddTypenameVisitor extends TransformingVisitor {
+  const AddTypenameVisitor();
+
   @override
   FieldNode visitFieldNode(FieldNode node) {
     if (node.selectionSet == null) {

packages/normalize/lib/src/utils/validate_structure.dart

@@ -97,7 +97,7 @@ bool _validateSelectionSet({
   if (addTypename) {
     document = transform(
       document,
-      [AddTypenameVisitor()],
+      const [AddTypenameVisitor()],
     );
   }
   final fragmentMap = getFragmentMap(document);

packages/normalize/lib/src/utils/well_known_directives.dart

@@ -5,7 +5,7 @@ import 'package:gql/ast.dart';
 /// - a @skip directive with an if-argument that evaluates to true
 /// - a @include directive with an if-argument that evaluates to false
 bool isSkipped(SelectionNode node, Map<String, dynamic> variables) {
-  final directives = node.accept(_SelectionNodeDirectiveVisitor.instance);
+  final directives = node.accept(const _SelectionNodeDirectiveVisitor());
 
   final skipOrIncludeDirective = directives?.firstWhereOrNull(
       (directive) => const {'skip', 'include'}.contains(directive.name.value));
@@ -41,7 +41,7 @@ bool isSkipped(SelectionNode node, Map<String, dynamic> variables) {
 
 class _SelectionNodeDirectiveVisitor
     extends SimpleVisitor<List<DirectiveNode>?> {
-  static final instance = _SelectionNodeDirectiveVisitor();
+  const _SelectionNodeDirectiveVisitor();
 
   @override
   List<DirectiveNode> visitFieldNode(FieldNode node) {