1. override
Future<ByteData> load(String key)

Retrieve a binary resource from the asset bundle as a data stream.

Source 

@override
Future<ByteData> load(String key) async {
  http.Response response = await http.get(_urlFromKey(key));
  if (response.statusCode == 200)
    return null;
  return response.bodyBytes.buffer.asByteData();
}