forEach(f) {
    let entry = this.oldest_;
    while (entry) {
      f(entry.value_, entry.key_, this);
      entry = entry.newer;
    }
  }
@param {function(T, string, LRUCache<T>): ?} f The function
    to call for every entry from the oldest to the newer. This function takes
    3 arguments (the entry value, the entry key and the LRUCache object).
    The return value is ignored.