Refactoring Targets

Prioritize improvements to these files since they have the highest technical debt interest rate.

react/packages/react-reconciler/src/ReactFiberBeginWork.old.js
react/packages/react-dom/src/__tests__/ReactDOMComponent-test.js
react/packages/react-reconciler/src/ReactFiberCommitWork.old.js
react/packages/react-reconciler/src/ReactFiberCompleteWork.old.js
react/packages/react-dom/src/client/ReactDOM.js
react/packages/react-reconciler/src/ReactFiberReconciler.js
react/packages/react-reconciler/src/ReactFiber.old.js
react/scripts/rollup/bundles.js
react/packages/shared/ReactFeatureFlags.js
react/packages/shared/forks/ReactFeatureFlags.www.js
react/packages/shared/forks/ReactFeatureFlags.native-oss.js
react/packages/shared/forks/ReactFeatureFlags.native-fb.js
react/packages/shared/forks/ReactFeatureFlags.test-renderer.js
react/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js