Simulate Sass 3.3 Maps data type when compiling with libsass (v2.0.0), still compiles successfully with Sass 3.3.

List Map

@function list-map($map, $map-key) {
  @each $val in $map {
    $key: nth($val, 1);
    $value: nth($val, 2);
    @if $map-key == $key {
      @return $value;
    }
  }
}