“Stranger Things: The First Shadow” has been runnin’ up that road, runnin’ up that hill for months on Broadway. Surprised observers feel like they’re in the Upside Down.  Wasn’t the play based on the Netflix series and produced by the streaming giant and Sonia Friedman Productions supposed to be a massive hit? Not so much. Last week, “First Shadow,” which only landed on Broadway in late March, played to just 75% capacity. The seven days before that? 65%.  For a production of this scale, a