src/vs/workbench/contrib/notebook/common/notebookEditorModel.ts/SimpleNotebookEditorModel/saveAs
class SimpleNotebookEditorModel:	async saveAs(target: URI): Promise<IUntypedEditorInput | undefined> {
		const newWorkingCopy = await this._workingCopyManager.saveAs(this.resource, target);
		if (!newWorkingCopy) {
			return undefined;
		}
		// this is a little hacky because we leave the new working copy alone. BUT
		// the newly created editor input will pick it up and claim ownership of it.
		return { resource: newWorkingCopy.resource };
	}