Add netstandard 2.0 target to Antlr3.Runtime

[zgabi]

Currently I'm receiving the following error in a .net core project:
error PS0264: Ambiguous match: need to map 'System.Runtime.Serialization.StreamingContext, System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e, Context=Runtime' and have several candidates: 'System.Runtime.Serialization.StreamingContext, Antlr3.Runtime, Version=3.5.0.2, Culture=neutral, PublicKeyToken=eb42632606e9261f, Context=Reference', 'System.Runtime.Serialization.StreamingContext, System.Runtime, Version=4.2.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, Context=Reference.

Because it is referencing the netstandard1.1 version of Antrl3.Runtime package which has a hack for the StreamingContext sturct:
https://github.com/antlr/antlrcs/blob/master/Runtime/Antlr3.Runtime/System/StreamingContext.cs

This hack is not needed in netstandard2.0

[sharwell]

error PS0264

What is this error? It's not being reported by the compiler.

[zgabi]

It is a PostSharp error code. It somehow checks all the classes and StreamingContext is duplicated, because it exists in the framework and in Antlr, too.

[sharwell]

I've marked this issue as an enhancement request, but keep in mind the type here is marked internal and should not be impacting your tools. I suggest filing a bug report on PostSharp to correct their logic, as the current implementation should not break execution behavior even for netstandard2.0.

[zgabi]

Thank you.

Btw: regaldless of this issue it is recommended (by Microsoft) to also target netstandard 2.0:
https://docs.microsoft.com/en-us/dotnet/standard/net-standard

However, targeting lower .NET Standard versions introduces a number of support dependencies. If your project targets .NET Standard 1.x, we recommend that you also target .NET Standard 2.0. This simplifies the dependency graph for users of your library that run on .NET Standard 2.0 compatible frameworks, and it reduces the number of packages they need to download.

For example the netstantard version of your package has a dependency to the NETStandard.Library, which has a lot of dependencies to System* packages in netstandard < 2.0