The SharePointOnline.Helper.dll, rebuilt with references to the v16 Microsoft.SharePoint.Client assemblies. Specifically, Microsoft.SharePoint.Client.dll and Microsoft.SharePoint.Client.Runtime.dll. SharePoint client assemblies version 16 are specifically for working against SharePoint Online. From my experience they work fine with SharePoint 2013 on-premise as well, however v15 originally existed for this purpose.
NOTE: The filename has ‘_.txt’ appended. This is only added as WordPress rejected upload of .dll files. Please “save as…” and remove the ‘_.txt’ from the filename.