I think we can build it however closed or open we would like it to be. The sharing of source code I suspect is more social than technical, therefore I don't expect much change.

Java Applets, Flash, SilverLight always was at a disadvantage, not being native to the browser runtime. Startup time and the need for a "plugin" hurt them. And they all kinda sucked in their own way. I hoped for a long time that Dart / native DartVM to Chrome could make a stride, but sadly they were fighting uphill battle against the sheer volume of JS developers resisting change. I'm convinced that it's never about the language[1], but the VM and it's abstraction layer. We need a layer that we can compile JavaScript(/HTML/CSS), Scala, Haskell, C#, F# etc. down to, not more languages.

I wonder how many man-years has been wasted on vertically aligning div's inside a div. :)

[1] Guy Steele, amazing talk on "Growing a Language", https://www.youtube.com/watch?v=_ahvzDzKdB0