function ListSelectionModel(opt_length) {
    this.length_ = opt_length || 0;
    // Even though selectedIndexes_ is really a map we use an array here to get
    // iteration in the order of the indexes.
    this.selectedIndexes_ = [];
  }
Creates a new selection model that is to be used with lists.

@param {number=} opt_length The number items in the selection.

@constructor
@extends {!cr.EventTarget}