flutter_svg/example/assets/simple/mask_with_use.svg

<svg viewBox="0 0 59 84"
    xmlns="http://www.w3.org/2000/svg"
    xmlns:xlink="http://www.w3.org/1999/xlink">
    <defs>
        <path id="a" d="M0 0h62v86H0z"/>
    </defs>
    <g transform="translate(0 -2)" fill="none" fill-rule="evenodd">
        <mask id="b" fill="#ddd"><use xlink:href="#a"/></mask>
    </g>
</svg>