function del (target, key) {
  if (isUndef(target) || isPrimitive(target)
  ) {
    warn(("Cannot delete reactive property on undefined, null, or primitive value: " + ((target))));
  }
  if (Array.isArray(target) && isValidArrayIndex(key)) {
    target.splice(key, 1);
    return
  }
  var ob = (target).__ob__;
  if (target._isVue || (ob && ob.vmCount)) {
    warn(
      'Avoid deleting properties on a Vue instance or its root $data ' +
      '- just set it to null.'
    );
    return
  }
  if (!hasOwn(target, key)) {
    return
  }
  delete target[key];
  if (!ob) {
    return
  }
  ob.dep.notify();
}
Delete a property and trigger change if necessary.