Hundreds of false errors in .NET SDK solution with multi-targeting projects

[AArnott]

Version Used:

VS 2017 RTW (dev15rtwsvc 26228.04)

Steps to Reproduce:

Disable NuGet package restore in the IDE to workaround NuGet/Home#4764

git clone https://github.com/aarnott/pinvoke
cd pinvoke
git checkout c5642c29ddd
.\build -Restore -Build
src\pinvoke.sln

Open BCrypt.cs (from the BCrypt project).
Opening Hid.cs and other files also does this.

Expected Behavior:

No errors

Actual Behavior:

The error list is full of false errors.

[davkean]

I think we'll probably want to take first look at this. I'll move it, and reopen if it's not us.

[davkean]

This issue was moved to dotnet/project-system#1696