"if the VM eventually becomes much faster than the dart2js path, Google can essentially force other browsers' hands by adopting Dart on Google sites"

Or all of the browser vendors could sit down and design a new language based on the experience gained from implementing Dart. This new more easily optimizable language can become a new web standard. After all this is what happened with SPDY and HTTP2.0. This "new language" could also just be a new version of Javascript with a few backwards incompatible changes.

One interesting claim of the DartVM authors is that they have achieved a 10x start up gain over Javascript when using script snapshots. If this turns out to be true, it will add significant pressure for other vendors to implement a similar feature. The only way to get this to work in Javascript is to make backwards incompatible changes. For example only allowing a static top-level structure, as in Dart.

I see the competition from Dart as being good for end users, as it adds pressure on Javascript to evolve. It's also interesting to see the V8 team porting a number of Dart's optimisations to V8.