ApiCompat misses several API differences

[tannergooding]

Whatever tool does the checks to ensure the CoreFX ref assemblies match up with the System.Private.Corelib signatures (I believe this is ApiCompat) misses a few differences.

Namely, dotnet/corefx#31528 and dotnet/corefx#31515 should have initially failed because the various classes had the following changes:

• Added a base class to many classes
• Added the new modifier to several methods which were purposefully hiding methods in a base class

CC. @ericstj, @weshaggard

[tannergooding]

Removed the issue with it not catching the sealed modifier, it is failing with that now, perhaps the PR got into a bad state.

[ericstj]

For the base class issue have a look at https://github.com/dotnet/buildtools/blob/master/src/ApiCompat/Rules/InheritanceHierarchyChangeTracker.cs. For historical reasons we were lenient here. If I had to guess we probably would ignore a base class removal if it didn't remove any instance members. We may want to change/parameterize that behavior. I bet we do still rely on it when doing comparisons between contracts and desktop.

For the latter we probably need to do something with IsNewSlot here: https://github.com/dotnet/buildtools/blob/master/src/ApiCompat/Rules/Compat/MembersMustExist.cs. I suspect we didn't due to historical leniency again. For this one I don't see a good reason not to just make it the default behavior. Folks can always baseline it if they are intentionally omitting new (for instance if they intentionally omitted a base class).

[ViktorHofer]

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