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

    let calledBack = false;
    let error = null;

    mm.preloadModule('b', 2);
    clock.tick(10);

    assertFalse('module "b" should be done loading', mm.isModuleLoading('b'));

    const d = mm.load('b');
    d.then(
        (ctx) => {
          calledBack = true;
        },
        (err) => {
          error = err;
        });

    clock.tick(1);
    assertTrue(calledBack);
    assertNull(error);
  }
Tests loading a module via load when the module is already
loaded.  The deferred's callback should be called immediately.