code: function validateExplicitKey(element, parentType) {
    if (element._store.validated || element.key != null) {
      return;
    }
    element._store.validated = true;
  
    warnAndMonitorForKeyUse(
      'Each child in an array or iterator should have a unique "key" prop.',
      element,
      parentType
    );
  }
docs: Warn if the element doesn't have an explicit key assigned to it.
  This element is in an array. The array could grow and shrink or be
  reordered. All children that haven't already been validated are required to
  have a "key" property assigned to it.
  
  @internal
  @param {ReactElement} element Element that requires a key.
  @param {*} parentType element's parent's type.