同步导入所有资源。
By default some assets can be imported asynchronously (e.g. scripts can be compiled in the background).
In some cases all importing needs to be synchronous; use this flag then. For example, when importing
a scripts + prefabs, scripts have to be fully compiled before Prefab is serialized, otherwise
it might get old variables.
另请参阅：AssetDatabase.ImportAsset、AssetDatabase.Refresh。