This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
} else {
        const iteratorFn = getIteratorFn(children);
        if (typeof iteratorFn === 'function') {
          const childrenIterator = iteratorFn.call(children);
          if (childrenIterator) {
            let step = childrenIterator.next();
            let i = 0;
            for (; !step.done; step = childrenIterator.next()) {
              if (!validateSuspenseListNestedChild(step.value, i)) {
                return;
              }
              i++;
            }
          }
        } else {
          console.error(
            'A single row was passed to a <SuspenseList revealOrder="%s" />. ' +
              'This is not useful since it needs multiple rows. ' +
              'Did you mean to pass multiple children or an array?',
            revealOrder,
          );
        }
      }
    }
  }
}
function initSuspenseListRenderState(
  workInProgress: Fiber,
  isBackwards: boolean,
  tail: null | Fiber,