function addMapEntry(map, pair) {
  // Don't return `map.set` because it's not chainable in IE 11.
  map.set(pair[0], pair[1]);
  return map;
}
Adds the key-value `pair` to `map`.

@private
@param {Object} map The map to modify.
@param {Array} pair The key-value pair to add.
@returns {Object} Returns `map`.