Should `async` be allowed as an attribute name?

[saschanaz]

[Exposed=Window,
 HTMLConstructor]
interface HTMLScriptElement : HTMLElement {
  // -- snip --
  [CEReactions] attribute boolean async;
  [CEReactions] attribute boolean defer;
  // -- snip --
};

The attribute async now has an invalid syntax after #720. Should it be just escaped with _async or should we add async to AttributeName?

cc @Ms2ger @bzbarsky

[bzbarsky]

The latter, imo.