if I remember correctly, Dart was announced end of 2011 and become available to "play with" almost immediately. Since couple month after announcement, I have not seen anyone mentioning it ever. In 'internet years', 2011 is ancient history - so I am not sure if "it's still early in Dart's life" apply.
I can see Google pushing it into Chrome, but I can not see Google replacing JS with it and I do not see any other browser except Chrome adopting it. Dart solves problem which no one has.