testLoad_concurrent() {
    const mm = getModuleManager({'a': [], 'b': [], 'c': []});
    mm.setConcurrentLoadingEnabled(true);
    mm.setLoader(createSuccessfulNonBatchLoader(mm));

    const calledBack = false;
    const error = null;

    mm.load('a');
    mm.load('b');
    assertEquals(2, requestCount);
    // Only time for one serialized download.
    clock.tick(5);

    assertTrue(mm.getModuleInfo('a').isLoaded());
    assertTrue(mm.getModuleInfo('b').isLoaded());
  }
Tests loading 2 modules asserting that the loads happen in parallel
in one unit of time.