One of the design goals: "Ensure that Dart delivers high performance on all modern web browsers and environments ranging from small handheld devices to server-side execution." Server-side execution? I thought Go was their systems programming language? It sure sounds like they're competing rather than complementing each other.
There's a lot of server side programming for which Go probably wouldn't be the ideal language. Room for more than 2 languages in the world I think :)
There's room, I just don't get why Google would promote two of its own very similar languages for the same purpose at the same time.