Analyzer rejects const constructor in class with `abstract` instance variable.

[lrhn]

Example:

abstract class C {
  abstract int x;
  const C();  // Error here.
}
abstract class D {
  int get x;
  set x(int _);
  const D();
}
void main() {}

is rejected with the message:

error
line 3 • Can't define a const constructor for a class with non-final fields. (view docs)
Try making all of the fields final, or removing the keyword 'const' from the constructor.

This is incorrect, since the class C does not have any non-final instance variables, or instance variables at all.
It has an abstract variable declaration, but that introduces an abstract getter and setter, which are not backed by an instance variable.

[bwilkerson]

@scheglov