Google web developers who want to deploy their apps need to use dart2js. Google web developers need to make sure their apps work in all modern browsers. If the Dart team wants to promote adoption even within Google (and they do) they need to support all modern browsers.
Dartium and dart2js already aren't quite in sync; numeric types work differently for example. But they're close enough that for most apps, you can develop in Dartium and deploy on dart2js and the language differences don't really bother you. The language differences between Dart-in-JavaScript and Dart-in-Dartium are a lot less than the differences between browsers that web developers deal with all the time.
Dartium and dart2js already aren't quite in sync; numeric types work differently for example. But they're close enough that for most apps, you can develop in Dartium and deploy on dart2js and the language differences don't really bother you. The language differences between Dart-in-JavaScript and Dart-in-Dartium are a lot less than the differences between browsers that web developers deal with all the time.