fix(60617): abstract modifier cannot be used with private a identifier

[a-tarasyuk]

Fixes #60617

[Andarist]

It would be great to add those 2 tests to check that the abstractness of those is still working OK:

abstract class A1 {
  abstract #x: number;

  getX() {
    return this.#x;
  }

  static B = class B extends A1 {
    #x = 5;
  };
}

abstract class A2 {
  abstract #x: number;

  getX() {
    return this.#x;
  }

  static B = class B extends A2 {
    // x stays not implemented
  };
}

[whzx5byb]

abstract class A {
    abstract #x: number;
    getX() {
        return this.#x;
    }
}

will be compiled to

class A {
    getX() {
        return this.#x;
    }
}

and throw a runtime error because an abstract member will be erased.

[RyanCavanaugh]

This needs the corresponding emit change (and test) to preserve #x; when we're seeing abstract #x;

[whzx5byb]

@RyanCavanaugh I don't think it's a good idea. Even if we preserve #x, the private property is still not "implementable", for example this code:

abstract class A {
  abstract #x: number;

  getX() {
    return this.#x;
  }

  static B = class B extends A {
    #x = 5;
  };
}

will be compiled to (if we preseve #x):

class A {
  #x;

  getX() {
    return this.#x;
  }

  static B = class B extends A {
    #x = 5;
  };
}

and if you call new A.B().getX() you will only get undefined, which is not the desired value.