lookForward = () => {
          const other = references[forwardI];
          if (other) {
            if (other.syncTime != null) {
              return other.syncTime + forwardAdd;
            }
            forwardAdd -= other.endTime - other.startTime;
            forwardI += 1;
          }
          return null;
        }
Look forwards 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 DOES count the original reference, but DOESN'T count the first
reference with a syncTime (as we approach it from behind).
@return {?number}