I'm on the Dart team.<p>The headline isn't very explicit, but I think this is pretty exciting. What this means is that we are taking the previous unsound optional type system and replacing it with a sound static type system.<p>Even though strong mode is sound and mandatory, the language still has much of the flexible feel it had before. The folks who designed the type system did a fantastic job on the inference engine. Having a few million lines of existing optionally-typed code that needed to gracefully migrate into strong mode certainly helped too.