This is the native code for creating MethodChannel for Notifications in Flutter

MainActivity.kt

package com.example.app

import androidx.annotation.NonNull
import io.flutter.embedding.android.FlutterActivity
import io.flutter.embedding.engine.FlutterEngine
import io.flutter.plugin.common.MethodChannel
import android.content.Context
import android.content.ContextWrapper
import android.content.Intent
import android.content.IntentFilter
import android.os.Build.VERSION
import android.os.Build.VERSION_CODES
import android.app.NotificationManager;
import android.app.NotificationChannel;
import android.net.Uri;
import android.media.AudioAttributes;
import android.content.ContentResolver;


class MainActivity: FlutterActivity() {
  private val CHANNEL = "example.com/notifications_channel" //The channel name you set in your main.dart file

  override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {
    super.configureFlutterEngine(flutterEngine)
    MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL).setMethodCallHandler {
      // Note: this method is invoked on the main thread.
      call, result ->

      if (call.method == "createNotificationChannel"){
        val argData = call.arguments as java.util.HashMap<String, String>
          val completed = createNotificationChannel(argData)
          if (completed == true){
              result.success(completed)
          }
          else{
              result.error("Error Code", "Error Message", null)
          }
      } else {
        result.notImplemented()
      }
    }

  }

    private fun createNotificationChannel(mapData: HashMap<String,String>): Boolean {
        val completed: Boolean
        if (VERSION.SDK_INT >= VERSION_CODES.O) {
            // Create the NotificationChannel
            val id = mapData["id"]
            val name = mapData["name"]
            val descriptionText = mapData["description"]
            val importance = NotificationManager.IMPORTANCE_HIGH
            val mChannel = NotificationChannel(id, name, importance)
            mChannel.description = descriptionText

            // Register the channel with the system; you can't change the importance
            // or other notification behaviors after this
            val notificationManager = getSystemService(NOTIFICATION_SERVICE) as NotificationManager
            notificationManager.createNotificationChannel(mChannel)
            completed = true
        }
        else{
            completed = false
        }
        return completed
    }
}