function buildGlamorSrcFromClassName(className) {
  return {[`data-${className}`]: ''}
}
Glamor's css function returns an object with the shape

{
  [`data-css-${hash}`]: '',
  toString() { return `css-${hash}` }
}

Whenever glamor's build function encounters an object with
this shape it just pulls the resulting styles from the cache.

note: the toString method is not needed to qualify the shape