extend(arr) {
    for (let i = 0, ii = arr.length; i < ii; ++i) {
      this.push(arr[i]);
    }
    return this;
  }
Add elements to the collection.  This pushes each item in the provided array
to the end of the collection.
@param {!Array<T>} arr Array.
@return {Collection<T>} This collection.
@api