It's ambiguous. "Not supported" might mean "we aren't going to write APIs in that language, and if you manage to hook in via FFI we don't promise stability. Also, since Fuchsia is security oriented, it probably will block various low level tricks for accessing system resources that don't use official APIs.

Reading between the lines, it seems Dart isn't better than Go from a performance perspective, but Dart is great for UIs so it is worth paying it's cost on an embedded device.