src/vs/workbench/api/common/extHostNotebookKernels.ts/NotebookCellExecutionTask/cellIndexToHandle
class NotebookCellExecutionTask:	private cellIndexToHandle(cellOrCellIndex: vscode.NotebookCell | undefined): number {
		let cell: ExtHostCell | undefined = this._cell;
		if (cellOrCellIndex) {
			cell = this._document.getCellFromApiCell(cellOrCellIndex);
		}
		if (!cell) {
			throw new Error('INVALID cell');
		}
		return cell.handle;
	}