Fat chance. It's already hellish to properly secure one JIT, a multiple-JIT browser would quickly look like Swiss cheese.
Face it: unless something miraculous happens (like Microsoft, Mozilla, Google and Opera all agreeing on a brand new language), javascript will likely remain the only choice for "in-browser" UI development. Unless the browser gets replaced wholesale, we're screwed like that.
Once Chrome gains enough users, they can choose to include Dartium (their Dart VM) in Chrome and choose to support that.
As a language Dart is not exciting. It doesn't tout fancy monads or functional data structures and so on, but it looks basic, simple and average. Kind of like what Java would be if it was made for the web.
So there is at least on contender out there to replace JS.
Mozilla is working on Rust. A very exciting language that actually has many fancy features, some borrowed from Erlang. Not sure if that will ever make into an in-browser language, but who knows.
Although I agree in spirit, I took a different design route in Ginger which supports multiple syntaxes and front-ends. By designing a generic syntax tree and defining its mapping into (a tight subset of) XML, I got away from the details of the back-end virtual machine. In fact Ginger has three very different back-ends, ported easily to another system (Poplog), and would be a days work to get it running in Java (badly - maybe a couple of weeks work to get it running well!)
Face it: unless something miraculous happens (like Microsoft, Mozilla, Google and Opera all agreeing on a brand new language), javascript will likely remain the only choice for "in-browser" UI development. Unless the browser gets replaced wholesale, we're screwed like that.