We did this in an ActionScript 3 compiler at my last job. The problem is that since code is dynamic by default, you still need the “fallback” operations mentioned in this post, because people can (and do) make use of dynamic operations and you need to preserve behaviour. Also, ActionScript’s type system isn’t expressive enough to actually get to full static typing—there is only one function type, for example, which says nothing about its parameter or return types. But in principle, and in a language that was designed to support it at the outset, it’s a good way to do typing.