Flutter is still beta and there is no OpenCV available for flutter unfortunately. But you can integrate the OpenCV native libs or sdk for both Android and IOS and invoke them through platform channels
platform channels
Hope that helped!
Flutter plugin package is avialable - https://pub.dartlang.org/packages/flutter_opencv
You can try this and it might be helpful.
Now there seems to be a pretty new opencv package providing a binding to OpenCV-4.x, published in April 2020.
However at the moment it is not a full binding yet.