flutter_svg/example/assets/simple/group_opacity_transform.svg

<svg viewBox="0 0 876.83 532.87"
  xmlns="http://www.w3.org/2000/svg">
  <g fill="black">
    <g opacity=".35" transform="translate(-50 -50)">
      <rect x="200" y="200" width="600" height="600" />
      <path d="m285.82 120.3v-47.75h-56"/>
    </g>
  </g>
</svg>