It's important to distinguish adopting a Dart VM from supporting Dart the language. Dart works fine in all modern browsers now by being compiled to JS. Doing that gives you the developer-time productivity boost of using Dart (which to me is the most compelling aspect of the language) while letting you ship your app to the entire web.

The VM is neat too, but from the user's perspective, it's an implementation detail. It's a really nice development tool, because it lets you run and debug your Dart code straight from source instead of trying to deal with generated code.

But I think it's relatively less important that the VM be shipped to end users. I think Dart can be a great, successful product without that happening.

It's not an issue. I've noticed dart2js performance is as good as hand-written js.

At least with dart2js I have just one "VM" to target - Google have sorted the x-browser issues for me. Imagine the peculiarities you'd find in Microsoft's implementation, for example.