すべてのアセットを同期的にインポートを行います
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.
関連項目: AssetDatabase.ImportAsset, AssetDatabase.Refresh.