Method operator[]

eval/public/builtin_func_test.cc:1160–1170

  int size() const override { return data_.size(); }

  absl::optional<CelValue> operator[](CelValue key) const override {
    absl::optional<T> raw_value = get_cel_value_(key);
    if (!raw_value) {
      return absl::nullopt;
    }
    auto it = data_.find(*raw_value);
    if (it == data_.end()) {
      return absl::nullopt;
    }
    return it->second;
  }

  absl::StatusOr<const CelList*> ListKeys() const override {
    return keys_.get();