/** For testing only. */
 Source withMockEstimateSizeBytes(Long estimateSizeBytes) {
   Source res = new Source(host, datasetId, query, namespace);
   res.mockSplitter = mockSplitter;
   res.mockEstimateSizeBytes = estimateSizeBytes;
   return res;
 }
 /** For testing only. */
 Source withMockSplitter(QuerySplitter splitter) {
   Source res = new Source(host, datasetId, query, namespace);
   res.mockSplitter = splitter;
   res.mockEstimateSizeBytes = mockEstimateSizeBytes;
   return res;
 }