anchor.diff:  
       gae_utils.CloudStorageLocation sourceLocation = await task.sourceLocation;
       if (lastSeenMd5 != null && sourceLocation.md5 == lastSeenMd5) {
  -      log.trace("Source cache hit: $lastSeenMd5");
  -      if (cachedSource == null) {
  +      log.trace("Source JSON cache hit: $lastSeenMd5");
  +      if (cachedSourceJSON == null) {
           throw "Invariant check failed: Cached key matched but source was null";
         }
   
  -      sourceJSON = cachedSource;
  +      sourceJSON = cachedSourceJSON;
       } else {
  +      var oldMd5 = lastSeenMd5;
  +
         sourceJSON = await
           gae_utils.CloudStorage.getFileContentsByLocation(sourceLocation)
               .transform(UTF8.decoder).join();
           lastSeenMd5 = sourceLocation.md5;
  -        cachedSource = sourceJSON;
  -        log.trace("Cache miss: Source acquired $lastSeenMd5");
  +        cachedSourceJSON = sourceJSON;
  +        log.trace("Cache miss (was: $oldMd5): Source JSON acquired $lastSeenMd5");
       }
   
       var sourceMap = JSON.decode(sourceJSON);
  
positive.code: 
  String workerFolder;
  const START_NAME = "worker_isolate.dart";
  
  // Worker properties.
  SendPort sendPort;
  
negative.code: 
                 if (task == null) {
                   log.info("Got null next ready task, sleeping");
                   await new Future.delayed(DELAY_BETWEEN_TASK_POLLS);
                   continue;
                 }
  
  