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.
this.previousWasTextNode = false;
    this.makeStaticMarkup = makeStaticMarkup;
  }
  read(bytes: number): string | null {
    if (this.exhausted) {
      return null;
    }
    var out = '';
    while (out.length < bytes) {
      if (this.stack.length === 0) {
        this.exhausted = true;
        break;
      }
      var frame: Frame = this.stack[this.stack.length - 1];
      if (frame.childIndex >= frame.children.length) {
        var footer = frame.footer;
        out += footer;
        if (footer !== '') {
          this.previousWasTextNode = false;
        }
        this.stack.pop();
        if (frame.tag === 'select') {
          this.currentSelectValue = null;
        }
        continue;
      }
      var child = frame.children[frame.childIndex++];
      if (__DEV__) {
        setCurrentDebugStack(this.stack);
      }