Taking a long view, it seems certain that both will co-exist for the next 5-7 years. But if developers see just how much better (in terms of complexity, user experience) that new web apps can be if written in Dart, then it deserves to get wider take up. There are already third-party websites for Dart appearing (e.g. Dartr.com0. Ultimately it’s down to web developers to judge which language to use.