Dart isn't perfect, and I think at the least would really benefit from Union types. There are some open issues related to these things though:

https://github.com/dart-lang/sdk/issues/4938 (Unions) https://github.com/dart-lang/sdk/issues/421 (Tuples)

Please feel free to contribute here! That said, it's certainly possible to work around these things. Dart has a tuple library (https://github.com/dart-lang/tuple) which covers a lot, and much of what you'd use Unions for can still be covered by accepting `Object` or `dynamic` parameter types and doing some typechecking.