flutter_svg/example/assets/simple/mask_with_use2.svg

<svg xmlns:xlink="http://www.w3.org/1999/xlink" height="130px" version="1.1"
    viewBox="-10 -10 120 120" width="130px" xmlns="http://www.w3.org/2000/svg">
    <defs>
        <path d="M10,35 A20,20,0,0,1,50,35 A20,20,0,0,1,90,35 Q90,65,50,95 Q10,65,10,35 Z"
            id="path-1" />
    </defs>

    <mask fill="white" id="mask-1">
        <use xlink:href="#path-1"></use>
    </mask>
         
    <polygon fill="orange" points="-10,110 110,110 110,-10 -10,-10" />
     
    <circle cx="50" cy="50" fill="green" mask="url(#mask-1)" r="50" />
</svg>