code: function keysIndexes(children, startIndex, endIndex) {
    var i, keys = Object.create(null), key;
    for (i = startIndex; i <= endIndex; ++i) {
      if (children[i]) {
        key = children[i].key;
        if (key !== undefined) {
          keys[key] = i;
        }
      }
    }
    return keys;
  }
docs: Returns indexes of keyed nodes.
  
  @param  Array  children An array of nodes.
  @return Object          An object of keyed nodes indexes.