[flow analysis] promote variable in a function expression if potentially final

[asashour]

void g(void Function() x) {}

void f(int? i) {
  int? x;
  if (i == null) return;
  x = i; // here x is not-nullable
  g(() {
    x.isEven; // 🔥 x is nullable
  });
}

Current behavior: on entering the function, x promotion is discarded, so it is not promoted to be not-nullable, because x is assigned (written).

The current proposed implementation is that on entering the closure, variables which are:

1. not written/assigned in any closure, and
2. not written after "entering" this closure

are considered final and so promotion is preserved.

[asashour]

https://dart-review.googlesource.com/c/sdk/+/277880

[lrhn]

The example above seems sound because there is no demoting assignment to x down-flow from the creation of the closure.
If there were, it would not be sound to assume that x still has the same type when the closure function is eventually called.

[lrhn]

Moving this to the language repo, since any change to inference is a language change.

[stereotype441]

Duplicate of #1536.