Issue https://github.com/flutter/flutter/issues/15953 it works:

AspectRatio(
  aspectRatio: 1,
  child: ClipRect(
    child: FittedBox(
      fit: BoxFit.cover,
      child: SizedBox(
        width: _controller!.value.previewSize.height,
        height: _controller!.value.previewSize.width,
        child: CameraPreview(_controller!),
      ),
    ),
  ),
)