src/vs/workbench/contrib/notebook/browser/notebookServiceImpl.ts/NotebookService/_onWillDisposeDocument
class NotebookService:	private _onWillDisposeDocument(model: INotebookTextModel): void {
		const modelData = this._models.get(model.uri);
		if (modelData) {
			this._onWillRemoveNotebookDocument.fire(modelData.model);
			this._models.delete(model.uri);
			modelData.dispose();
			this._onDidRemoveNotebookDocument.fire(modelData.model);
		}
	}