export function createFiberFromTypeAndProps(
  type: any, // React$ElementType
  key: null | string,
  pendingProps: any,
  owner: null | Fiber,
  mode: TypeOfMode,
  expirationTime: ExpirationTime,
): Fiber {
  let fiber;

  let fiberTag = IndeterminateComponent;
  // The resolved type is set if we know what the final type will be. I.e. it's not lazy.
  let resolvedType = type;
  if (typeof type === 'function') {
    if (shouldConstruct(type)) {
      fiberTag = ClassComponent;
    }
  } else if (typeof type === 'string') {
    fiberTag = HostComponent;
  } else {
  //...

这里判断了 typeof type === function
但下面只判断了 ClassComponent 的类型：通过 shouldConstruct(type)
那 FunctionComponet在哪判断的？它确实就是个普通函数，返回的 ReactElement，这种也是合法的子节点呀