function testExec_UsesExistingResult() {
  var inputRelation = lf.proc.Relation.fromRows([], [j.getName()]);
  var aggregatedColumn = lf.fn.count();
  var aggregationResult = 100;
  inputRelation.setAggregationResult(aggregatedColumn, aggregationResult);
  var childStep = new lf.proc.NoOpStep([inputRelation]);
  var aggregationStep = new lf.proc.AggregationStep([aggregatedColumn]);
  aggregationStep.addChild(childStep);

  return aggregationStep.exec().then(function(relations) {
    assertEquals(
        aggregationResult,
        relations[0].getAggregationResult(aggregatedColumn));
  });
}
Tests that AggregationStep is using existing aggregation result
(pre-calculated by previous steps).
@return {!IThenable}