What are the best ways of handling versioning and experimental features in a programming lang?

• Python has the __future__ module: python.org/dev/peps/pep-0…
• JS has 'use strict'
• Handle it outside of the language. Rust (?), JS (script vs. module)

Dart's approach seems really well thought out: github.com/dart-lang/sdk/… (thanks @mraleph)

One thing I am curious about is why experiments can't be enabled by the @ dart comments.

E.g. `// @ dart=2.9+non-nullable` doesn't seem to be valid, only `// @ dart=2.9`.