This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
tag: RootTag,
  isStrictMode: boolean,
  concurrentUpdatesByDefaultOverride: null | boolean,
): Fiber {
  let mode;
  if (tag === ConcurrentRoot) {