I guess you should have a look at the language spec, there are answers to some of your questions and concerns. (http://www.dartlang.org/docs/spec/dartLangSpec.pdf)

For example: " Dart supports optional typing based on interface types. The type system is unsound, due to the covariance of generic types. This is a deliberate choice (and undoubtedly controversial). Experience has shown that sound type rules for generics fly in the face of programmer intuition. It is easy for tools to provide a sound type analysis if they choose, which may be useful for tasks like refactoring."