We are not going to migrate away from JavaScript (it would be impossible). We are figuring out how to use JavaScript without JavaScript's own warts.
http://www.quirksmode.org/blog/archives/2011/10/dart_or_why_...
The Dart team is very aware that it is highly unlikely that the DartVM will be put into any browser except Chrome (and there is no time-table on when that will happen). That is why their Dart2js compiler is seeing so much attention from the team.
Dart, to me, is Google's new GWT, but trying to align more with the web as we know it today (instead of hiding the DOM from its users). Even if Dart is only ever cross-compiled to Javascript and used that way in production, I would prefer to code in Dart than Javascript for anything that requires more than a few lines of javascript to implement.
Nobody thought JS and Ajax was a serious thing until Google Maps came about and everyone's jaws dropped. Despite all the academic or theoretical benefits async requests it might have had, they didn't become popular until a killer application came about.
I am also thinking Dart for Android? I wish they had Python for android, but oh well, I understand it was much more of a political thing, but I can see Dart+VM being that platform.
Seems sensible for google to have some options similar to Java for both Android, and their in-house server-side code. (Oracle anyone?)
...even better performance? See the light-blue line on the graph in the OP.