Improved the logic of adding default assembly references for Desktop CLR CSX scripts

[filipw]

This PR improves the way we are adding default assembly references for Desktop CLR CSX scripts. We need to make sure we add references to all assemblies type forwarders might point to when working with a Desktop CLR script (for .NET Core scripts, all references should be provided via the project model).

The main difference was so far we references mscorlib.dll and System.Core.dll but missed System.dll.

Fixes dotnet/vscode-csharp#1581

[DustinCampbell]

Why not use an IAssemblyLoader?

[filipw]

and here I was, so happy that I used a local function... 😋
you are right though

I also noticed it needs a rebase

[david-driscoll]

Is there any point of adding any unit tests at all?

[filipw]

Is there any point of adding any unit tests at all?

Good point.
I'll check - let's hold off on the merge.

I think we should be able to have a test based on using the Regex type (dotnet/vscode-csharp#1581) that would fail on the old OmniSharp code and would work here.

[filipw]

Actually the test infrastructure has it's own logic for default references, that's why this issue never showed up in tests before
https://github.com/OmniSharp/omnisharp-roslyn/blob/dev/tests/TestUtility/TestHelpers.cs#L26

I think this should be good to merge for now then.
In the future we can eliminate that little duplication of default references between the test infrastructure with the project system.

[david-driscoll]

Sounds good to me. @DustinCampbell ?

[DustinCampbell]

Fine by me!

[filipw]

everything is green. I think this is good to merge - no reason to keep it hanging around anymore