Add additional properties callback in Inspector Serialization Delegate (#45531)

* Add additional properties callback in Inspector Serialization Delegate

* Rename _SerializationDelegate to InspectorSerializationDelegate and add test

* Fix indentation

* Remove trailing whitespace

* Handle case when addAdditionalPropertiesCallback returns null

* Improve docs and minor renames

* Improve docs

* Improve documentation

packages/flutter/test/widgets/widget_inspector_test.dart

@@ -2599,6 +2599,76 @@ class TestWidgetInspectorService extends Object with WidgetInspectorService {
       visitChildren(detailedChildren);
       expect(appBars.single, isNot(contains('children')));
     }, skip: !WidgetInspectorService.instance.isWidgetCreationTracked()); // Test requires --track-widget-creation flag.
+
+    testWidgets('InspectorSerializationDelegate addAdditionalPropertiesCallback', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        MaterialApp(
+          title: 'Hello World!',
+          home: Scaffold(
+            appBar: AppBar(
+              title: const Text('Hello World!'),
+            ),
+            body: Center(
+              child: Column(
+                children: const <Widget>[
+                  Text('Hello World!'),
+                ],
+              ),
+            ),
+          ),
+        )
+      );
+      final Finder columnWidgetFinder = find.byType(Column);
+      expect(columnWidgetFinder, findsOneWidget);
+      final Element columnWidgetElement = columnWidgetFinder
+        .evaluate()
+        .first;
+      final DiagnosticsNode node = columnWidgetElement.toDiagnosticsNode();
+      final InspectorSerializationDelegate delegate =
+        InspectorSerializationDelegate(
+          service: service,
+          summaryTree: false,
+          includeProperties: true,
+          addAdditionalPropertiesCallback:
+            (DiagnosticsNode node, InspectorSerializationDelegate delegate) {
+              final Map<String, Object> additionalJson = <String, Object>{};
+              final Object value = node.value;
+              if (value is Element) {
+                additionalJson['renderObject'] =
+                  value.renderObject.toDiagnosticsNode().toJsonMap(
+                    delegate.copyWith(subtreeDepth: 0),
+                  );
+              }
+              additionalJson['callbackExecuted'] = true;
+              return additionalJson;
+            },
+        );
+      final Map<String, Object> json = node.toJsonMap(delegate);
+      expect(json['callbackExecuted'], true);
+      expect(json.containsKey('renderObject'), true);
+      expect(json['renderObject'], isA<Map<String, dynamic>>());
+      final Map<String, dynamic> renderObjectJson = json['renderObject'];
+      expect(renderObjectJson['description'], startsWith('RenderFlex'));
+
+      final InspectorSerializationDelegate emptyDelegate =
+        InspectorSerializationDelegate(
+          service: service,
+          summaryTree: false,
+          includeProperties: true,
+          addAdditionalPropertiesCallback:
+            (DiagnosticsNode node, InspectorSerializationDelegate delegate) {
+              return null;
+            },
+        );
+      final InspectorSerializationDelegate defaultDelegate =
+        InspectorSerializationDelegate(
+          service: service,
+          summaryTree: false,
+          includeProperties: true,
+          addAdditionalPropertiesCallback: null,
+        );
+      expect(node.toJsonMap(emptyDelegate), node.toJsonMap(defaultDelegate));
+    });
   }
 }