testPreloadDeferredWhenNotLoaded() {
    const mm = getModuleManager({'a': []});
    mm.setLoader(createSuccessfulNonBatchLoader(mm));

    let calledBack = false;

    const d = mm.preloadModule('a');
    d.addCallback((ctx) => {
      calledBack = true;
    });

    // First load should take five ticks.
    assertFalse('module "a" should not be loaded yet', calledBack);
    clock.tick(5);
    assertTrue('module "a" should be loaded', calledBack);
  }
Tests that preloading a module calls back the deferred object.