code: function makeSetMarkup(markup) {
    // NOTE: Null values reduce hidden classes.
    return {
      type: 'SET_MARKUP',
      content: markup,
      fromIndex: null,
      fromNode: null,
      toIndex: null,
      afterNode: null
    };
  }
docs: Make an update for setting the markup of a node.
  
  @param {string} markup Markup that renders into an element.
  @private