code: function makeInsertMarkup(markup, afterNode, toIndex) {
    // NOTE: Null values reduce hidden classes.
    return {
      type: 'INSERT_MARKUP',
      content: markup,
      fromIndex: null,
      fromNode: null,
      toIndex: toIndex,
      afterNode: afterNode
    };
  }
docs: Make an update for markup to be rendered and inserted at a supplied index.
  
  @param {string} markup Markup that renders into an element.
  @param {number} toIndex Destination index.
  @private