Allow for more type checking in the for-in statement

[bwilkerson]

If the definition of the for-in statement were re-written to be

---

T n0 = e.iterator;
while (n0.moveNext()) {
   finalConstVarOrType? id = n0.current;
   s
}

where 'T' is the static type of 'e.iterator'.

---

We would be able to detect when the type of 'n0.current' is not assignable to 'id'. See issue #18534 for background.

[lrhn]

This would not require e to implement Iterable, it can still be anything that has an "iterator" getter.
However, if the static type of e happens to be "Iterable<SomeType>", then the static type of "e.iterator" would be "Iterator<SomeType>", and using that as the type of "iterator" would man that "iterator.current" has static type "SomeType" - and we would be able to warn instead of just hinting.

[sigurdm]

Any news on this?
Is the spec not allowed to be changed in a way that gives more warnings, but same unchecked behavior?
I think many people stumble on this, and think this has to do with the unsoundness of dart's type system (and not just a problem in the specification).

[munificent]

We do this now: #18534