We (https://www.blossom.io) are using Dart alongside JavaScript (CoffeeScript) right now.

You can call JavaScript code from Dart and vice versa. https://www.dartlang.org/articles/js-dart-interop/

This was a great pragmatic stepping stone for us as we moved from a JavaScript only code base to a much saner development experience thanks to Dart.

Related write-up: http://news.dartlang.org/2013/04/why-blossom-is-switching-to...