Analyzer allows writing to late final instance variables

[stereotype441]

The following program is accepted by the analyzer but rejected by the CFE:

class C {
  late final int x = 0;
  C(int value) {
    x = value;
  }
}
main() {
  print(C(1).x);
}

I believe this is the relevant spec text:

A non-local late variable declaration D implicitly induces a getter
into the enclosing scope. It also induces an implicit setter iff one of the
following conditions is satisfied:

• D is non-final.
• D is late, final, and has no initializing expression.

Since the declaration of x is late, final, and has an initializing expression, it meets neither of these conditions, so there should be no implicit setter and the assignment x = value should be illegal.

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/161471