I created a new Android and iOS/NETStandard Xamarin.Forms application with Visual Studio 2017 15.5.4 and immediately got an exclamation mark on Dependencies node in Common project. Trying to build the solution I got like three errors for start. They were saying something along
Since I had no idea what that message wanted to tell me I went checking for references and I quickly saw that nor Android nor iOS had any NuGet reference present albeit the packages were listed as installed. So, no packages for some reason. I quickly dismissed the possibility of bad "new Xamarin.Forms project" template - Internet would be screaming in such case.
error : Assets file '[APPPATH]\obj\project.assets.json' not found. Run a NuGet package restore to generate this file.
After tinkering a bit, I saw that Package Manager window was saying that it can't find a reference to NuGet Source for one of my sources. Which was correct, but I wasn't using that source at all.
It turns out that once disabling that (faulty) source, NuGet was able to restore the packages and project was building.
Actually it was a combination of two problems combined - one was that missing source that prevented NuGet to download packages and the other problem was that the required NuGet packages weren't cached locally on my computer - if they were I believe it would work regardless of the faulty source.