> My takeaway is this: Doing something quick and dirty for a first draft and improve it later on often leads to better results in the long run than planing and over-engineering a solution beforehand, because you can start refining details much earlier or throw away bad approaches without investing too much time.
I like Dart because it supports what you've described really really well. Optional type annotations enable you to don't worry about types when you do rapid prototyping.
Once your understanding of a good solution solidifies you can sprinkle type annotations over function signatures and wherever it makes sense to get great tooling support and to fortify core parts of your code base :)
In a nutshell it enables project lifecycles many companies go through many times (something written in Ruby eventually might have to be rewritten in Java for performance and tooling/collaboration support) yet being able to stick to one language which removes a ton of complexity.