code: @Override
    public final FunctionType getSuperClassConstructor() {
      checkArgument(isConstructor() || isInterface());
      ObjectType maybeSuperInstanceType = getPrototype().getImplicitPrototype();
      if (maybeSuperInstanceType == null) {
        return null;
      }
      return maybeSuperInstanceType.getConstructor();
    }
docs: Given a constructor or an interface type, get its superclass constructor or {@code null} if
  none exists.