Method attributeChangedCallback

src/core/a-node.js:67–74

* Handle mixin.

(attr, oldVal, newVal)

   * Handle mixin.
   */
  attributeChangedCallback (attr, oldVal, newVal) {
    // Ignore if `<a-node>` code is just updating computed mixin in the DOM.
    if (newVal === this.computedMixinStr) { return; }

    if (attr === 'mixin' && !this.isMixin) {
      this.updateMixins(newVal, oldVal);
    }
  }

 /**
  * Returns the first scene by traversing up the tree starting from and