code: function attr(loc, reference, attrName, value) {
    if (plainAttributes.indexOf(attrName) != -1) {
      return sourceNode(loc, [reference, '.', attrName, ' = ', value, ';']);
    } else {
      return sourceNode(loc, [reference, '.setAttribute(', esc(attrName), ', ', value, ');']);
    }
  }
docs: Generate source nodes for attribute.
  @param {Object} loc
  @param {string} reference
  @param {string} attrName
  @param {string} value
  @returns {SourceNode}