Both Dart and Go can be run in the browser or on the server, so you can share code between client and server regardless of which language you choose, so long as you don't mix them.

Well, maybe. I have not kept up with GopherJS; how are they doing on code size? And while you can certainly run Dart on an App Engine Managed VM, there is no free tier like with Go.

Not sure about GopherJS. I'm confused about how App Engine's pricing model relates to whether or not Dart is well-suited to server tasks?

Well, it's relevant to those of us who are using App Engine. Where else can I can I set up a service for a hobby project without a monthly charge if it's barely used?