Dart has a broadly similar server side role to Go (the approach is very different) in that it falls between C++ and currently popular dynamic languages, plus it has a client-side role where it aims to be a better JS.

Obviously on the server side it overlaps with Go, but Google has the resources to do multiple efforts in parallel in areas they think are important to find what works best with real world experience.

In the history footnotes, unless Google manages to convince other browser vendors to adopt it.