Summary

This PR introduces a development-time warning for custom React renderers that are still using the deprecated `reconciler.flushSync` API. It aims to improve developer experience by providing clearer error messages and guiding them towards the correct replacement APIs, without affecting production behavior.

Changes

Impact Areas

Files Changed

• packages/react-reconciler/src/ReactFiberReconciler.js
• packages/react-reconciler/src/ReactFiberHostConfig.js
• packages/react-dom/src/client/ReactDOMRoot.js