I'm biased, being on the Dart team, but I think Dart is a much nicer language that Java, so I would choose Dart for a new project. GWT might still be a good choice though if you have a large existing investment in Java that you can repurpose in the client. Many enterprises have hundreds of thousands or millions of lines of Java code that aren't going anywhere anytime soon.
So, should you think about Dart? I guess that depends on how ready or willing you are to migrate your code away form Java. If you were thinking about rewriting in JavaScript, then I'd definitely say consider Dart. If you're happy with Java and GWT, then you can keep going that direction.