* Creates a [`NotebookCellExecutionTask`](#NotebookCellExecutionTask). Should only be called by a kernel. Returns undefined unless requested by the active kernel.
* @param uri The [uri](#Uri) of the notebook document.
* @param index The index of the cell.
* @param kernelId The id of the kernel requesting this run task. If this kernel is not the current active kernel, `undefined` is returned.