code: public JSTypeExpression getParameterType(String parameter) {
      if (info == null || info.parameters == null) {
        return null;
      }
      return info.parameters.get(parameter);
    }
docs: Gets the type of a given named parameter.
  @param parameter the parameter's name
  @return the parameter's type or {@code null} if this parameter is not
  defined or has a {@code null} type