Flutter how to make a lower border round with invisible other parties

[baileigh]

Design needs to round down the bottom of the hat with invisible other parties.

As you can see from the image, the bottom of the hat is a thin round line. The problem I've encountered is if I ask the container to borderRadius and try to hide any of the parties to the border, there's a mistake, boxDecoration Only if all sides can round the line border'а one colour, one thickness and one thickness style solid

    return Container(
      width: size.width,
      height: size.height * 0.3,
      decoration: BoxDecoration(
        color: theme.backgroundColor,
        border: Border(
          bottom: BorderSide(color: theme.primaryColor, width: 2),
        ),
        borderRadius: BorderRadius.only(
          bottomLeft: Radius.circular(25),
          bottomRight: Radius.circular(25),
        ),
      ),
      child: Center(
        child: Image.asset('assets/screens/a/images/header.png',
            fit: BoxFit.fill),
      ),
    );

Text of error:

======== Exception caught by rendering library =====================================================
The following assertion was thrown during paint():
A borderRadius can only be given for a uniform Border.
The following is not uniform:

BorderSide.color

BorderSide.width

BorderSide.style

Googled the subject, recommended the crutches from the extra view, but there was no normal answer.

[Anderson]

I made the following decision. For border The container has a shadow.

    return Container(
      width: size.width,
      height: size.height * 0.3,
      decoration: BoxDecoration(
        color: theme.backgroundColor,
        boxShadow: <BoxShadow>[
          new BoxShadow(
            color: theme.primaryColor,
            offset: Offset(0, 1),
          ),
        ],
        borderRadius: BorderRadius.circular(30),
      ),
      child: Center(
        child: Image.asset('assets/screens/a/images/header.png',
            fit: BoxFit.fill),
      ),
    );

The result was satisfactory.