src/vs/workbench/api/common/extHostTypes.ts/NotebookCellOutput/isNotebookCellOutput
class NotebookCellOutput:	static isNotebookCellOutput(candidate: any): candidate is vscode.NotebookCellOutput {
		if (candidate instanceof NotebookCellOutput) {
			return true;
		}
		if (!candidate || typeof candidate !== 'object') {
			return false;
		}
		return typeof (<NotebookCellOutput>candidate).id === 'string' && isArray((<NotebookCellOutput>candidate).items);
	}