updateEntities() {
    var entity;
    var entities;
    var i;
    if (!this.sceneEl) {
      return;
    }
    entities = this.sceneEl.querySelectorAll('[mixin~=' + this.id + ']');
    for (i = 0; i < entities.length; i++) {
      entity = entities[i];
      if (!entity.hasLoaded || entity.isMixin) {
        continue;
      }
      entity.mixinUpdate(this.id);
    }
  }
For entities that already have been loaded by the time the mixin was attached, tell
those entities to register the mixin and refresh their component data.