addInstance(instancingId, instancedSprite) {
    let instances = this._instancesById[instancingId];

    // generate list of instances as needed
    if (instances == null) {
      instances = this._instancesById[instancingId] = [];
    }

    if (!_.contains(instances, instancedSprite)) {
      // add instance to list
      instances.push(instancedSprite);

      // flag this instancing as dirty
      this.setInstancesDirty(instancingId);
    }
  }
Adds a sprite as an instance. Instanced sprites all reference the same (dynamic) texture, but may be different sizes.
@param {String} instancingId
@param {cc.Sprite} instancedSprite