This changes our NuGet download and commits us to package reference
elements.

Create the Packages.props and FixedPackages.props.  These are the files
which will contain our MSBuild mapping from packag name to package
version

regression test for  https://github.com/dotnet/roslyn/issues/19119

enabled one test for a fixed bug.

Fix #18905 - No error reported for an expression bodied accessor in abstract event (changes after review)

The trait NetCore was being unconditionally passed as the trait option to our unit tests.  That mean all tests were being filtered
to those which had the trait NetCore which is a very small number.  Fixiyng it to only be passed when running the net core suites

Break was inserted via bad copy / paste in the following

https://github.com/dotnet/roslyn/commit/970cf1762ed422df17a7a22646be0d178d1c139f

Upgrade Roslyn to consume newer Microsoft.Net.RoslynDiagnostics

Disabling flaky tests failed in last 24hr.

My motivation for doing this is to pick up the new PublicAPI checker
that understands type forwards. In doing so I disabled a bunch of new
warnings that are either broken or not really applicable to Roslyn.
One new warning (don't rethrow exceptions with throw ex;) seemed useful
enough to keep on so I left it on and fixed the one offense in the
codebase.

Disabling some ReferenceHighlighting tests due to flakiness

Fix setup authoring for Microsoft.DiaSymReader.Native

Fix tests

Add workaround for AsyncSemaphore disposal problems

Conflicts:
	docs/compilers/CSharp/Compiler Breaking Changes - post VS2017.md
	src/Compilers/CSharp/Portable/Errors/ErrorCode.cs
	src/Compilers/VisualBasic/Portable/VBResources.resx

See: https://github.com/dotnet/roslyn/issues/19059

disabling ShowGenericTypes3

The test is flakey, fails in microbuild

Use MicrosoftSHA2 cert for signing

Add dev15.3-preview1 branch badge

This is a workaround for #18563. See #19042 for a follow-up task to remove
this workaround.