[104] Missing todo in step 4

[michdud]

When adding _BackdropState to backdrop.dart, it seems like there should be a comment about the ExcludeSemantics widget in _buildStack() above widget.backLayer:

  // TODO: Add BuildContext and BoxConstraints parameters to _buildStack (104)
  Widget _buildStack() {
    return Stack(
    key: _backdropKey,
      children: <Widget>[
        // (new) TODO: Wrap backLayer in an ExcludeSemantics widget (104)
        widget.backLayer,
        widget.frontLayer,
      ],
    );
  }

This todo shows up in step 6 but isn't anywhere else in the tutorial before this that I can see.

[rami-a]

Thanks for finding this issue. I've updated the codelab with this change!