robertjohnson.work/docs/assets/plugins/font-awesome/scss/_rotated-flipped.scss

// Rotated & Flipped Icons
// -------------------------

.#{$fa-css-prefix}-rotate-90  { @include fa-icon-rotate(90deg, 1);  }
.#{$fa-css-prefix}-rotate-180 { @include fa-icon-rotate(180deg, 2); }
.#{$fa-css-prefix}-rotate-270 { @include fa-icon-rotate(270deg, 3); }

.#{$fa-css-prefix}-flip-horizontal { @include fa-icon-flip(-1, 1, 0); }
.#{$fa-css-prefix}-flip-vertical   { @include fa-icon-flip(1, -1, 2); }
.#{$fa-css-prefix}-flip-horizontal.#{$fa-css-prefix}-flip-vertical { @include fa-icon-flip(-1, -1, 2); }

// Hook for IE8-9
// -------------------------

:root {
  .#{$fa-css-prefix}-rotate-90,
  .#{$fa-css-prefix}-rotate-180,
  .#{$fa-css-prefix}-rotate-270,
  .#{$fa-css-prefix}-flip-horizontal,
  .#{$fa-css-prefix}-flip-vertical {
    filter: none;
  }
}