Due to the fact that Flutter does not allow large file copies yet (at least that is what seems to be the case by January of 2019), I created this Plugin.

The following code is normally used to copy a small file from the assets folder to the App Documents folder where it can be used further...

Directory directory = await getApplicationDocumentsDirectory();
var dbPath = join(directory.path, "app.db");
if (FileSystemEntity.typeSync(dbPath) == FileSystemEntityType.notFound) {
  ByteData data = await rootBundle.load("assets/my_db_file.db");
  List<int> bytes = data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes);
  await File(dbPath).writeAsBytes(bytes);
}

However, I found that the above code does not work for large files ! Therefore the motivation for this plugin (i.e. to make the file-copy natively for each platform).

The Plugin copies a large File natively ...

• from the native-Bundle (iOS)...
• from the assets-folder (Android)...

--> ...and places it to your Application Documents Folder

(of course, it does the copying only if the file does not already exist at the Application Documents Folder location [same name]).

The plugin method created is called copyLargeFile

The return-value of the copyLargeFile method delivers the fullNamePath of the copied (or already existing) large File inside the Application Documents Folder.

The following preparation is needed as for the native worlds respectively. (i.e. in both worlds, you need to provide the large-file inside the corresponding native Bundle or Assets-folder...)

iOS:

• open your Flutter-project's /ios/Runner.xcworkspace in Xcode
• inside Runner, create a new Group and call it for example "Bundle"
• inside Bundle (or whatever you called your Group), drag and drop your large File (make sure to select "Copy items if needed")
• (optional): to check if all is correct for iOS - go to your Flutter project's ios/Runner/ Folder and see if the added large File is there...

Android:

• open your Flutter-project's /android/my_flutter_project_name_android.iml in Android Studio
• inside app/app/src/main/assets/ folder drag and drop your large file (create assets folder if it is not already there)
• (optional): to check if all is correct for Android - go to your Flutter project's android/src/main/assets/ Folder and see if the added large File is there...

dependencies:
  flutter:
    sdk: flutter
  large_file_copy: ^0.0.1

(hint: the version might be different in the meantime. Make sure to include the newest version of the package plugin).

import 'package:large_file_copy/large_file_copy.dart';

  @override
  void initState() {
    super.initState();
    initPlatformState();
  }

  // Platform messages are asynchronous, so we initialize in an async method.
  Future<void> initPlatformState() async {
    String fullPathName;

    try {
      platformVersion = await LargeFileCopy.platformVersion;
      print(platformVersion);
      fullPathName = await LargeFileCopy("testy.txt").copyLargeFile;
      print(fullPathName);
    } on PlatformException {
      print('Failed to copy the Large File.');
      return;
    }

    if (!mounted) return;

    print(fullPathName); // this is the location of your large-file now. Use it for anything in your Flutter app.
  }

The path to the file is given by the fullPathName return value of the native LargeFileCopy("myLargeFileName.db").copyDB method.