How can I include native code in Flutter to display a KML Layer on Google Map?

Question

I've been looking into running native code in my Flutter app to display a KML Layer on my Google Map and while I do have the app running native code (at least on Android), I am having difficulty with getting the KML stuff to work.

I have created a MethodChannel in my Flutter class to run the native code and this works fine. Please see below.

// Run java code for KML Campus Map Overlay
  Future<void> _showCampusMap() async {
    const platform = MethodChannel('uk.ac.manchestermaps/kmlLayer');
    try {
      final campusMapOverlay =
          await platform.invokeMethod('retrieveFileFromUrl');
      print(campusMapOverlay);
    } on PlatformException catch (error) {
      print(error);
    }
  }

I am taking some code that I used on the Android only pre-alpha version of the app, so I know it works, but I have 4 errors.

In my MainActivity java file,I have these package.

import android.os.Bundle;
import io.flutter.app.FlutterActivity;
import io.flutter.plugins.GeneratedPluginRegistrant;
import io.flutter.plugin.common.MethodCall;
import io.flutter.plugin.common.MethodChannel;
import io.flutter.plugin.common.MethodChannel.Result;
import io.flutter.plugin.common.MethodChannel.MethodCallHandler;

import org.xmlpull.v1.XmlPullParserException;
import android.os.AsyncTask;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.URL;

and the MainActivity class consists of this.

public class MainActivity extends FlutterActivity {
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    GeneratedPluginRegistrant.registerWith(this);

    new MethodChannel(getFlutterView(), "**<MY CHANNEL>**").setMethodCallHandler(new MethodCallHandler() {
      @Override
      public void onMethodCall(MethodCall call, MethodChannel.Result result) {

        if (call.method.equals("retrieveFileFromUrl")) {
          retrieveFileFromUrl();
          result.success("KMLLayer Retrieved");
        }

      }
    });
  }

  private void retrieveFileFromUrl() {
    new DownloadKmlFile("**<REMOTE KML FILE>**")
        .execute();
  }

  private class DownloadKmlFile extends AsyncTask<String, Void, byte[]> {
    private final String mUrl;

    public DownloadKmlFile(String url) {
      mUrl = url;
    }

    protected byte[] doInBackground(String... params) {

      try {
        InputStream is = new URL(mUrl).openStream();
        // Log.d(TAG, "doInBackground: " + mUrl.toString());
        ByteArrayOutputStream buffer = new ByteArrayOutputStream();
        int nRead;
        byte[] data = new byte[16384];
        while ((nRead = is.read(data, 0, data.length)) != -1) {
          buffer.write(data, 0, nRead);
        }
        buffer.flush();
        return buffer.toByteArray();
      } catch (IOException e) {
        e.printStackTrace();
      }
      return null;
    }

    protected void onPostExecute(byte[] byteArr) {
      try {
        KmlLayer kmlLayer = new KmlLayer(mMap, new ByteArrayInputStream(byteArr), getApplicationContext());
        kmlLayer.addLayerToMap();
        // moveCameraToKml(kmlLayer);
      } catch (XmlPullParserException e) {
        e.printStackTrace();
      } catch (IOException e) {
        e.printStackTrace();
      }
    }
  }
}

Here are my errors

Running Gradle task 'assembleDebug'...

.....\MainActivity.java:73: error: cannot find symbol
        KmlLayer kmlLayer = new KmlLayer(mMap, new ByteArrayInputStream(byteArr), getApplicationContext());
        ^

  symbol:   class KmlLayer

  location: class MainActivity.DownloadKmlFile
.....\MainActivity.java:73: error: cannot find symbol
        KmlLayer kmlLayer = new KmlLayer(mMap, new ByteArrayInputStream(byteArr), getApplicationContext());
                                ^

  symbol:   class KmlLayer

  location: class MainActivity.DownloadKmlFile
.....\MainActivity.java:73: error: cannot find symbol
        KmlLayer kmlLayer = new KmlLayer(mMap, new ByteArrayInputStream(byteArr), getApplicationContext());
                                         ^

  symbol:   variable mMap
  location: class MainActivity.DownloadKmlFile

3 errors

I have looked around, but can't find anything to help this this specific case as I don't think a lot of people are doing on it. I know KML support has been request for the google_maps_flutter package, but it seems to have gone quiet.

Any help with this would be very much appreciated, as it is pretty central to the app I am developing. Without this, the app is next to useless.

Thanks