Dart equivalent of signal.butter

I need to port the below python code to dart but I can't seem to find the equivalent to the signal.butter() function.

 def pan_tompkins_detector(self, unfiltered_ecg, MWA_name='cumulative'):
        """
        Jiapu Pan and Willis J. Tompkins.
        A Real-Time QRS Detection Algorithm. 
        In: IEEE Transactions on Biomedical Engineering 
        BME-32.3 (1985), pp. 230–236.
        """
        
        maxQRSduration = 0.150 #sec
        f1 = 5/self.fs
        f2 = 15/self.fs

        b, a = signal.butter(1, [f1*2, f2*2], btype='bandpass')

        filtered_ecg = signal.lfilter(b, a, unfiltered_ecg)        

        diff = np.diff(filtered_ecg) 

        squared = diff*diff

        N = int(maxQRSduration*self.fs)
        mwa = MWA_from_name(MWA_name)(squared, N)
        mwa[:int(maxQRSduration*self.fs*2)] = 0

        mwa_peaks = panPeakDetect(mwa, self.fs)

        return mwa_peaks

This is what I already came up with:

    double maxQRSduration = 0.150;
    int fs = 250;
    double f1 = 5/fs;
    double f2 = 15/fs;
    dynamic b= firwin(3, Array([f1*2, f2*2]), pass_zero: false, fs: 250);

The firwin function does not seem to do the job. Are there any other dart packages or functions that are the same as the signal.butter function in python?