On an internal chat, I asked:
> Is there a reason I don't understand why `flutter` should still
support "unsound" null-safety modes?
@jonahwilliams:
> I don't believe its possible to have non-null safe flutter code.
---
There is quite a bit of code and branches, so to keep the PRs
understandable/approachable, this first PR removes the ability to have
the `DevFS` implementations for the web SDK choose between sound and
unsound null safety, and removes the mapping accordingly.
The enums still exist, but can be cleaned up in a follow-up PR (you'd
want to rename to remove "sound" as well, ideally).
2d30cae396