code: Map<String, JobState.DatasetState> getDatasetStatesByUrns() {
      return ImmutableMap.copyOf(this.datasetStatesByUrns.or(Maps.<String, JobState.DatasetState>newHashMap()));
    }
docs: Get a {@link Map} from dataset URNs (as being specified by {@link ConfigurationKeys#DATASET_URN_KEY} to
  {@link JobState.DatasetState} objects that represent the dataset states and store {@link TaskState}s
  corresponding to the datasets.
  
  @see JobState#createDatasetStatesByUrns().
  
  @return a {@link Map} from dataset URNs to {@link JobState.DatasetState}s representing the dataset states