code: private Node parseContextTypeExpression(JsDocToken token) {
      if (token == JsDocToken.QMARK) {
        return newNode(Token.QMARK);
      } else {
        return parseBasicTypeExpression(token);
      }
    }
docs: ContextTypeExpression := BasicTypeExpression | '?'
  For expressions on the right hand side of a this: or new: