constructor ({ data }) {
    this.data = data
    // Stores thangType with slug or original as the key.
    this.loadedThangTypes = new Map()
    // Stores the thangType loading promise with slug or original as key.
    this.loadingThangTypes = new Map()
  }
Loader loads resources and stores them in a Map.
Thus the loader caches resources that have already been requested.