Follow up from https://github.com/facebook/react/pull/11616.

Here's one I found. These two invariants:

https://github.com/facebook/react/blob/7e7127387ba64f57299eaff8656542cea6fb3a2d/packages/react-reconciler/src/ReactFiberClassComponent.js#L330-L343

are in a function that's behind a DEV block:

https://github.com/facebook/react/blob/7e7127387ba64f57299eaff8656542cea6fb3a2d/packages/react-reconciler/src/ReactFiberClassComponent.js#L447-L449

I don't think this is intentional, but need to verify.

There may be more. I'll use this issue to track while https://github.com/facebook/react/pull/11616 preserves existing behavior.
return '';
      } else if (!React.isValidElement(nextChild)) {
        const nextChildren = toArray(nextChild);
        const frame: Frame = {
          domNamespace: parentNamespace,
          children: nextChildren,
          childIndex: 0,
          context: context,
          footer: '',
        };
        if (__DEV__) {
          ((frame: any): FrameDev).debugElementStack = [];
        }
        this.stack.push(frame);
        return '';
      } else if (
        ((nextChild: any): ReactElement).type === REACT_FRAGMENT_TYPE
      ) {
        const nextChildren = toArray(
          ((nextChild: any): ReactElement).props.children,
        );
        const frame: Frame = {
          domNamespace: parentNamespace,
          children: nextChildren,
          childIndex: 0,
          context: context,
          footer: '',
        };
        if (__DEV__) {
          ((frame: any): FrameDev).debugElementStack = [];