code: function computeUniqueAsyncExpiration() {
      var currentTime = recalculateCurrentTime();
      var result = computeAsyncExpiration(currentTime);
      if (result <= lastUniqueAsyncExpiration) {
        // Since we assume the current time monotonically increases, we only hit
        // this branch when computeUniqueAsyncExpiration is fired multiple times
        // within a 200ms window (or whatever the async bucket size is).
        result = lastUniqueAsyncExpiration + 1;
      }
      lastUniqueAsyncExpiration = result;
      return lastUniqueAsyncExpiration;
    }
docs: Creates a unique async expiration time.