Earlier quoted context omitted.
It compiles to JavaScript. The VM and dart2js are MIT licensed. It's not the same situation.
And who is responsible for keeping the DartVM/Dart2JS behaviour in sync ? Google being the ONLY committers don't exactly have a motivation to.
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.