code: function () {
      var collisionEntities;
      var data = this.data;
      var el = this.el;
  
      if (!data.collisionEntities) {
        this.collisionEntities = [];
        return;
      }
  
      collisionEntities = [].slice.call(el.sceneEl.querySelectorAll(data.collisionEntities));
      this.collisionEntities = collisionEntities;
  
      // Update entity list on attach.
      this.childAttachHandler = function childAttachHandler (evt) {
        if (!evt.detail.el.matches(data.collisionEntities)) { return; }
        collisionEntities.push(evt.detail.el);
      };
      el.sceneEl.addEventListener('child-attached', this.childAttachHandler);
  
      // Update entity list on detach.
      this.childDetachHandler = function childDetachHandler (evt) {
        var index;
        if (!evt.detail.el.matches(data.collisionEntities)) { return; }
        index = collisionEntities.indexOf(evt.detail.el);
        if (index === -1) { return; }
        collisionEntities.splice(index, 1);
      };
      el.sceneEl.addEventListener('child-detached', this.childDetachHandler);
    }
docs: Run `querySelectorAll` for `collisionEntities` and maintain it with `child-attached`
  and `child-detached` events.