getSheetIndex = function getSheetIndex() {
  return react_jss_esm_index++;
}
Global index counter to preserve source order.
As we create the style sheet during componentWillMount lifecycle,
children are handled after the parents, so the order of style elements would
be parent->child. It is a problem though when a parent passes a className
which needs to override any childs styles. StyleSheet of the child has a higher
specificity, because of the source order.
So our solution is to render sheets them in the reverse order child->sheet, so
that parent has a higher specificity.

We start at [Number.MIN_SAFE_INTEGER] to always insert sheets from react-jss first before any
sheet which might be inserted manually by the user.