src/vs/workbench/api/common/extHostNotebookDocument.ts/ExtHostCell/setOutputItems
class ExtHostCell:	setOutputItems(outputId: string, append: boolean, newOutputItems: extHostProtocol.NotebookOutputItemDto[]) {
		const newItems = newOutputItems.map(extHostTypeConverters.NotebookCellOutputItem.to);
		const output = this._outputs.find(op => op.id === outputId);
		if (output) {
			if (!append) {
				output.items.length = 0;
			}
			output.items.push(...newItems);
		}
	}