code: public com.sun.javadoc.Type superclassType() {
          if (asClassDoc().isInterface()) {
              return null;
          }
          Type sup = env.types.supertype(type);
          return TypeMaker.getType(env,
                                   (sup != type) ? sup : env.syms.objectType);
      }
docs: Return the class type that is a direct supertype of this one.
  Return null if this is an interface type.