* Given a request and cached response, this figures out whether to use the network, the cache, or
 * both.
 *
 * Selecting a cache strategy may add conditions to the request (like the "If-Modified-Since" header
 * for conditional GETs) or warnings to the cached response (if the cached data is potentially
 * stale).
 */
class CacheStrategy internal constructor(
  /** The request to send on the network, or null if this call doesn't use the network. */