message: Add comment to clarify Element.extends
diff: @@ -23,6 +23,10 @@ export class ScannedElement extends ScannedElementBase {
  tagName?: string;
  className?: string;
  superClass?: ScannedReference;
  +
  + /**
  + * For customized built-in elements, the tagname of the superClass.
  + */
  extends?: string;
  constructor() {
  @@ -44,6 +48,10 @@ export class Element extends ElementBase implements Feature {
  tagName?: string;
  className?: string;
  superClass?: Reference;
  +
  + /**
  + * For customized built-in elements, the tagname of the superClass.
  + */
  extends?: string;
  constructor() {
  