When you navigate in .Net Framework project (with JetBrains decompiler on Ctrl+Click) ReSharper can decompile GAC assemblies on-the-fly and shows implementation.
The same behavior on .Net Core shows stubs only from C:\Program Files\dotnet\sdk\NuGetFallbackFolder\microsoft.netcore.app\2.0.0\ref\netcoreapp2.0\System.Runtime.dll.
ReSharper 2017.2.1
I've asked that on Resharper support forum (because this problem also bothers me every day) and they confirmed (as far as I understand at least) that this is a bug and created an issue here. So as far as I can tell from that - you cannot fix that yourself and have to just wait until they fix it (issue has "Major" priority so there is chance it will happen in not that far future).
Update: this bug has been marked as fixed in version 2017.3.
