eclare attributes to observe via
   * the standard `static get observedAttributes()`, implement `_propertiesChanged`
   * on the class, and then call `MyClass.createPropertiesForAttributes()` once
   * on the class to generate property accessors for each observed attribute
   * prior to instancing.  Last, call `this._flushProperties()` once to enable
   * the accessors.
   *
   * Any `observedAttributes` will automatically be
   * deserialized via `attributeChangedCallback` and set to the associated
   * pr