How to use a local audio file instead of uri in speech to text API in Flutter?

I'm using the Google Speech To Text API from the googleapis package. But I didn't found any documentation(For dart & flutter) explaining how to use a local audio file present in the application assets folder as audio data while sending a RecognizeRequest.fromJson. I want to know how I can use a local file in place of audio content in _json in code. Thanks in advance.

    final httpClient = await clientViaServiceAccount(_credentials, _scopes);
    try {
      final speech2Text = SpeechApi(httpClient);

      final _json = {
        "config": {
          "encoding": "FLAC",
          "sampleRateHertz": 16000,
          "languageCode": "en-US",
          "enableWordTimeOffsets": false
        },
        "audio": {"uri": "gs://cloud-samples-tests/speech/brooklyn.flac"}
      };
      final _recognizeRequest = RecognizeRequest.fromJson(_json);
      await speech2Text.speech.recognize(_recognizeRequest).then((response) {
        for (var result in response.results) {
          print(result.toJson());
        }
      });
    } finally {
      httpClient.close();
    }
  }

I finally managed to do it by looking at the example of this google_speech package.

1. Add the audio file as an asset in the pubsepec.yaml:

assets:
- assets/brooklyn.flac

2. Then copy the file from an asset:

Future<void> _copyFileFromAssets(String name) async {
  var data = await rootBundle.load('assets/$name');
  final directory = await getApplicationDocumentsDirectory();
  final path = directory.path + '/$name';
  await File(path).writeAsBytes(
      data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes));
}

3. Then get the content of the audio:

Future<List<int>> _getAudioContent(String name) async {
  final directory = await getApplicationDocumentsDirectory();
  final path = directory.path + '/$name';
  if (!File(path).existsSync()) {
    await _copyFileFromAssets(name);
  }
  return File(path).readAsBytesSync().toList();
}

4. Now Encode the content to Base64:

final audio = await _getAudioContent('brooklyn.flac');
String audio64 = base64Encode(audio);

5. Pass the encoded string as a content string in the audio section:

final _json = {
   "config": {
      "encoding": "FLAC",
      "sampleRateHertz": 16000,
      "languageCode": "en-US",
      "enableWordTimeOffsets": false
      },
      // "audio": {"uri": "gs://cloud-samples-tests/speech/brooklyn.flac"}
   "audio": {"content": audio64},
   };

I hope this helps someone with a similar issue.