It compiles to JavaScript. The VM and dart2js are MIT licensed. It's not the same situation.
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.
So, yeah, Google has a motivation to keep it up to date.
And, then, sure, Dart2JS stops being important to Google because its unimportant to how Dart code performs on the web. But its unimportant to Google because its unimportant to anyone, since it won't matter to users, either.
We've seen that game before, played by microsoft.
Other browser vendors know this: Dart is never gonna fly outside of Google's walled garden.