lookBackward = () => {
          const other = references[backwardI];
          if (other) {
            if (other != reference) {
              backwardAdd += other.endTime - other.startTime;
            }
            if (other.syncTime != null) {
              return other.syncTime + backwardAdd;
            }
            backwardI -= 1;
          }
          return null;
        }
Look backwards one reference at a time, summing all durations as we
go, until we find a reference with a syncTime to use as a basis.
This DOESN'T count the original reference, but DOES count the first
reference with a syncTime (as we approach it from ahead).
@return {?number}