I'm just concerned that Google (being both the origin of Dart and a prominent developer of webapps) will spend most of their development time writing Dart apps

I'm just concerned that Google (being both the origin of Dart and a prominent developer of webapps) will spend most of their development time writing Dart apps running on their Dart VM and slowly begin to neglect (remember, doesn't necessarily have to be malicious) the performance of their JS output.