“Tomorrowland” is Brad Bird’s Jeremiad against the dystopia that is modern culture, with its yen for zombie apocalypses, environmental catastrophes and the 24 hours of fear telecast by cable news. It’s a movie for the “dreamers,” the ones like its teenage heroine Casey (Britt Robertson). She’s the only kid in class who asks the obvious, when confronted with lectures on nuclear proliferation, the unstable politics of much of the world and global climate change.