code: public TypeElement overriddenClass(ExecutableElement ee) {
          TypeMirror type = overriddenType(ee);
          return (type != null) ? asTypeElement(type) : null;
      }
docs: Return the class that originally defined the method that
  is overridden by the current definition, or null if no
  such class exists.
  
  @return a TypeElement representing the superclass that
  originally defined this method, null if this method does
  not override a definition in a superclass.