[mythz]

The main thing I miss from Swift is its extensibility letting you extend primitive types like Arrays or Strings, this lets you easily add LINQ-like capabilities even tho Swift doesn't ship with many, see: https://github.com/mythz/swift-linq-examples

Dart comes with a rich functional and Stream API's on collections so most of the time you wont need to extend it (same LINQ examples in Dart: https://github.com/dartist/101LinqSamples), but unfortunately you can't extend built-in (or external) types which means you need to use top-level methods which can break-up the flow of your beautiful expression flows. Extensions is also important for minimizing unnecessary abstraction and maximizing re-usability since you can extend without being forced to create foreign wrapper API's.

IMO Extensions are one of the most important traits for keeping a minimal friction-free language, something I see Go and Swift benefiting a lot from.

Dart does have powerful mixins (https://www.dartlang.org/articles/mixins/) but you can only mixin into classes you define (i.e. not built-in or external types).

But otherwise I find Dart's optional-typing a tonne more productive than Swift's strict type inference / generic enforcement, Dart also has a rich consistent library, integrated package manager, static analyzer and overall better development experience.

Although there's little overlap as Swift is mainly for creating iOS/OSX desktop apps and Dart is focused on rich Web Apps. Basically the only times when you'd be able to choose one or the other is in command-line scripts, I'd still prefer to use Dart for.