Project-system: Cleaning a solution or project shows warnings on all dependencies

[BillHiebert]

1. Create a new ASP.NET Core Web application targeting .NET Core 2.0 (version doesn't matter, repros with 1.0 and 1.1 too)
2. Build, all dependencies should be valid
3. Clean Solution. All the dependency nodes show the warning. Presumably this is because the asserts file has been removed? This seems like a bug to me.
4. Restore solution. Dependency nodes still show warning even though restore succeeds.
5. Build solution. Build succeeds but dependency node still shows the warnings.

This is with VS build 27309.3006.d15Stg

[BillHiebert]

I am seeing a lot of scenarios where the dependencies nodes show the warnings even though everything is fine. Sometimes just closing and re-opening a solution shows this.

[Pilchie]

@natidea does this look like another instance of the dataflow race?

[natidea]

Confirmed this no longer repros after applying race condition fix: https://github.com/dotnet/project-system/pull/3167