Today, I was looking into a way of getting the assemblies / dlls used by the SharePoint Online environment and surprisingly I found it that any user can download the assemblies / dlls from any of the O365 / SharePoint online environment without authentication.
ex: https://contoso.sharepoint.com/_vti_bin/microsoft.sharepoint.dll
I tried the same in our SharePoint online environment and I was able to download most of the dlls.
I have also attached one of the (Version : 16.0.2811.1219).
Unless it is a bug, it is a shortcut to get the latest version of the assemblies from your tenant. We have already twitted about this (https://twitter.com/PlanetWilson/status/464825498783465472) and awaiting more details from the community.