If a `@NgModule` has a `bootstrap` property, `PlatformRef.bootstrapModule` /
`PlatformRef.bootstrapModuleFactory` will automatically bootstrap the components
listed in there.
If such a property does not exist, `PlatformRef.bootstrapModule` /
`PlatformRef.bootstrapModuleFactory` will try to call the method `ngDoBootstrap(appRef: ApplicationRef)` on the module class.
Otherwise an error is reported.