Add warning for property assignment using private setter in non-final class

[eric-milles]

Consider the following:

class C {
  def m() { x = null }
  private void setX(x) {}
}
class D extends C /*implements Map*/ {
  /*
  @Delegate Map m
  void setX(x) { }
  void set(String name, value) { }
  */
}
new D().m()

The expression "x = null" will not make use of the private setter of C if one of the items in D is uncommented. "setX(null)" is safe or changing the visibility of setX to at least package-private or enabling static compilation for the call site prevents the interception as well. A warning could help bring awareness to this possibility.