Make `handleJsonEvent` public so that it can be overridden internally. (#139531)

packages/flutter_tools/lib/src/debug_adapters/flutter_adapter.dart

@@ -472,7 +472,7 @@ class FlutterDebugAdapter extends FlutterBaseDebugAdapter with VmServiceInfoFile
   }
 
   /// Handles incoming JSON events from `flutter run --machine`.
-  void _handleJsonEvent(String event, Map<String, Object?>? params) {
+  void handleJsonEvent(String event, Map<String, Object?>? params) {
     params ??= <String, Object?>{};
     switch (event) {
       case 'daemon.connected':
@@ -646,7 +646,7 @@ class FlutterDebugAdapter extends FlutterBaseDebugAdapter with VmServiceInfoFile
     final Object? params = payload['params'];
     final Object? id = payload['id'];
     if (event is String && params is Map<String, Object?>?) {
-      _handleJsonEvent(event, params);
+      handleJsonEvent(event, params);
     } else if (id != null && method is String && params is Map<String, Object?>?) {
       _handleJsonRequest(id, method, params);
     } else if (id is int && _flutterRequestCompleters.containsKey(id)) {