This causes a deprecation warning to be produced each time a list is used by a map function. To test, I made a simple sass file:

```scss
$lmap: a 1, b 2, c 3, d 4;
a {
  @each $key in map-keys($lmap) {
    #{$key}: map-get($lmap, $key);
  }
}
```

This produced 5 deprecation warnings:

```
DEPRECATION WARNING: Passing lists of pairs to map-keys is deprecated and will
be removed in future versions of Sass. Use Sass maps instead. For details, see
http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#maps.
        on line 5 of list_as_map.scss
DEPRECATION WARNING: Passing lists of pairs to map-get is deprecated and will
be removed in future versions of Sass. Use Sass maps instead. For details, see
http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#maps.
        on line 6 of list_as_map.scss
DEPRECATION WARNING: Passing lists of pairs to map-get is deprecated and will
be removed in future versions of Sass. Use Sass maps instead. For details, see
http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#maps.
        on line 6 of list_as_map.scss
DEPRECATION WARNING: Passing lists of pairs to map-get is deprecated and will
be removed in future versions of Sass. Use Sass maps instead. For details, see
http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#maps.
        on line 6 of list_as_map.scss
DEPRECATION WARNING: Passing lists of pairs to map-get is deprecated and will
be removed in future versions of Sass. Use Sass maps instead. For details, see
http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#maps.
        on line 6 of list_as_map.scss
```

I think a deprecation warning should only be produced the first time a list is used as a map. It's a LOT of noise otherwise.