<===> input.scss
test {
  filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#223344', endColorstr='#112233',GradientType=0 );
}

@mixin opacity($opacity) {
    opacity: $opacity / 100;
    filter: alpha(opacity=$opacity);
}
<===> output.css
test {
  filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#223344', endColorstr='#112233',GradientType=0 );
}

<===> output-dart-sass.css
test {
  filter: progid:DXImageTransform.Microsoft.gradient( startColorstr="#223344", endColorstr="#112233",GradientType=0 );
}