This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
function createFiberFromScope(
  scope: ReactScope,
  pendingProps: any,
  mode: TypeOfMode,
  lanes: Lanes,
  key: null | string,
) {
  const fiber = createFiber(ScopeComponent, pendingProps, key, mode);
  fiber.type = scope;
  fiber.elementType = scope;
  fiber.lanes = lanes;
  return fiber;
}
function createFiberFromProfiler(
  pendingProps: any,
  mode: TypeOfMode,
  lanes: Lanes,
  key: null | string,
): Fiber {
  if (__DEV__) {
    if (typeof pendingProps.id !== 'string') {
      console.error(
        'Profiler must specify an "id" of type `string` as a prop. Received the type `%s` instead.',
        typeof pendingProps.id,
      );
    }
  }
  const fiber = createFiber(Profiler, pendingProps, key, mode | ProfileMode);
  fiber.elementType = REACT_PROFILER_TYPE;
  fiber.lanes = lanes;