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!),
),
),
),
)