function stackSet(key, value) {
  var data = this.__data__;
  if (data instanceof ListCache) {
    var pairs = data.__data__;
    if (!Map || (pairs.length < LARGE_ARRAY_SIZE - 1)) {
      pairs.push([key, value]);
      this.size = ++data.size;
      return this;
    }
    data = this.__data__ = new MapCache(pairs);
  }
  data.set(key, value);
  this.size = data.size;
  return this;
}
Sets the stack `key` to `value`.

@private
@name set
@memberOf Stack
@param {string} key The key of the value to set.
@param {*} value The value to set.
@returns {Object} Returns the stack cache instance.