Type annotations provide inline documentation for your fellow developers. The toolchain can also read the type annotations and give you early warnings, refactoring support, and runtime type assertion errors.
Dart's types are there when you need them, but don't get in your way with too much ceremony or burden. This message was well received at the conference, based on the feedback that I got. Good stuff!
You can check out the slides from my Dart talk at FluentConf or views all the Dart presentations and slides on dartlang.org. Enjoy!