Support netcoreapp3.1 TFM

[JunTaoLuo]

Addresses dotnet/aspnetcore#13581.

[Pilchie]

LGTM though not sure about the pack exclusion.

[pranavkm]

Of the test failures:

Build contains unexpected warnings: /Users/vsts/agent/2.155.1/work/1/s/.dotnet/sdk/3.1.100-preview1-014024/Sdks/NuGet.Build.Tasks.Pack/build/NuGet.Build.Tasks.Pack.targets(198,5): warning NU5131: References were found in the nuspec, but some reference assemblies were not found in both the nuspec and ref folder. Add the following reference assemblies: [/private/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/Razor/4rzeiucd.iix/ClassLibrary/ClassLibrary.csproj]\n/Users/vsts/agent/2.155.1/work/1/s/.dotnet/dotnet msbuild /nr:false /bl /p:RunningAsTest=true /p:MicrosoftNETCoreApp31PackageVersion=3.1.0-preview1.19459.38 /p:MicrosoftNetCompilersToolsetPackageVersion=3.4.0-beta1-19461-06 /p:RuntimeAdditionalRestoreSources=/Users/vsts/agent/2.155.1/work/1/s/src/Razor/test/Microsoft.NET.Sdk.Razor.Test/../testapps/TestPackageRestoreSource /p:_RazorBuildServerPipeName=\"f9e32036-56ee-4817-8939-936c5423bf24\" /t:Pack /p:Configuration=Debug \n\nMicrosoft (R) Build Engine version 16.3.0-preview-19414-01+bbd9cfeca for .NET Core\nCopyright (C) Microsoft Corporation. All rights reserved.\n\n/Users/vsts/agent/2.155.1/work/1/s/.dotnet/sdk/3.1.100-preview1-014024/MSBuild.dll -maxcpucount -verbosity:m /bl /nr:false /p:RunningAsTest=true /p:MicrosoftNETCoreApp31PackageVersion=3.1.0-preview1.19459.38 /p:MicrosoftNetCompilersToolsetPackageVersion=3.4.0-beta1-19461-06 /p:RuntimeAdditionalRestoreSources=/Users/vsts/agent/2.155.1/work/1/s/src/Razor/test/Microsoft.NET.Sdk.Razor.Test/../testapps/TestPackageRestoreSource /p:_RazorBuildServerPipeName=f9e32036-56ee-4817-8939-936c5423bf24 /p:Configuration=Debug /t:Pack ./ClassLibrary.csproj\n You are using a preview version of .NET Core. See: https://aka.ms/dotnet-core-preview\n ClassLibrary -> /private/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/Razor/4rzeiucd.iix/ClassLibrary/bin/Debug/netcoreapp3.1/ClassLibrary.dll\n ClassLibrary -> /private/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/Razor/4rzeiucd.iix/ClassLibrary/bin/Debug/netcoreapp3.1/ClassLibrary.Views.dll\n Successfully created package '/private/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/Razor/4rzeiucd.iix/ClassLibrary/bin/Debug/ClassLibrary.1.0.0.nupkg'.\n/Users/vsts/agent/2.155.1/work/1/s/.dotnet/sdk/3.1.100-preview1-014024/Sdks/NuGet.Build.Tasks.Pack/build/NuGet.Build.Tasks.Pack.targets(198,5): warning NU5131: References were found in the nuspec, but some reference assemblies were not found in both the nuspec and ref folder. Add the following reference assemblies: [/private/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/Razor/4rzeiucd.iix/ClassLibrary/ClassLibrary.csproj]\n/Users/vsts/agent/2.155.1/work/1/s/.dotnet/sdk/3.1.100-preview1-014024/Sdks/NuGet.Build.Tasks.Pack/build/NuGet.Build.Tasks.Pack.targets(198,5): warning NU5131: [/private/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/Razor/4rzeiucd.iix/ClassLibrary/ClassLibrary.csproj]\n\n\n\nExit Code:0

I don't know what this means.

Build_ImplicitCSharp8_NullableEnforcement_WarningsDuringBuild_NoBuildServer: AFAIK, Roslyn uses C# 8.0 exclusively for netcoreapp3.0 (not netcoreapp3.0 or newer). We could skip this test and file a follow up item: https://github.com/dotnet/roslyn/blob/master/src/Compilers/Core/MSBuildTask/Microsoft.CSharp.Core.targets#L6-L10

[JunTaoLuo]

@pranavkm I'm seeing the following failure RazorSdk_DoesNotBuildViewsForNetCoreApp11Projects:

Microsoft.AspNetCore.Razor.Design.IntegrationTests.Assert+BuildWarningsException : Build contains unexpected warnings: CSC : warning CS1701: Assuming assembly reference 'System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' used by 'System.Console' matches identity 'System.Runtime, Version=4.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' of 'System.Runtime', you may need to supply runtime policy [/tmp/Razor/2aypdfr1.3ko/SimpleMvc11/SimpleMvc11.csproj]
Program.cs(10,13): warning CS1701: Assuming assembly reference 'System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' used by 'System.Console' matches identity 'System.Runtime, Version=4.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' of 'System.Runtime', you may need to supply runtime policy [/tmp/Razor/2aypdfr1.3ko/SimpleMvc11/SimpleMvc11.csproj]
/__w/1/s/.dotnet/dotnet msbuild /nr:false /bl /p:RunningAsTest=true /p:MicrosoftNETCoreApp31PackageVersion=3.1.0-preview1.19459.38 /p:MicrosoftNetCompilersToolsetPackageVersion=3.4.0-beta1-19461-06 /p:RuntimeAdditionalRestoreSources=/__w/1/s/src/Razor/test/Microsoft.NET.Sdk.Razor.Test/../testapps/TestPackageRestoreSource /p:_RazorBuildServerPipeName=\"1151764f-0cbd-4f13-9ecb-3682f9ccfbbc\" /t:Build /p:Configuration=Release 

Microsoft (R) Build Engine version 16.3.0-preview-19414-01+bbd9cfeca for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

/__w/1/s/.dotnet/sdk/3.1.100-preview1-014024/MSBuild.dll -maxcpucount -verbosity:m /bl /nr:false /p:RunningAsTest=true /p:MicrosoftNETCoreApp31PackageVersion=3.1.0-preview1.19459.38 /p:MicrosoftNetCompilersToolsetPackageVersion=3.4.0-beta1-19461-06 /p:RuntimeAdditionalRestoreSources=/__w/1/s/src/Razor/test/Microsoft.NET.Sdk.Razor.Test/../testapps/TestPackageRestoreSource /p:_RazorBuildServerPipeName=1151764f-0cbd-4f13-9ecb-3682f9ccfbbc /p:Configuration=Release /t:Build ./SimpleMvc11.csproj
 You are using a preview version of .NET Core. See: https://aka.ms/dotnet-core-preview
CSC : warning CS1701: Assuming assembly reference 'System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' used by 'System.Console' matches identity 'System.Runtime, Version=4.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' of 'System.Runtime', you may need to supply runtime policy [/tmp/Razor/2aypdfr1.3ko/SimpleMvc11/SimpleMvc11.csproj]
Program.cs(10,13): warning CS1701: Assuming assembly reference 'System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' used by 'System.Console' matches identity 'System.Runtime, Version=4.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' of 'System.Runtime', you may need to supply runtime policy [/tmp/Razor/2aypdfr1.3ko/SimpleMvc11/SimpleMvc11.csproj]
 SimpleMvc11 -> /tmp/Razor/2aypdfr1.3ko/SimpleMvc11/bin/Release/netcoreapp1.1/SimpleMvc11.dll



Exit Code:0

[pranavkm]

Could you skip it? .NET Core 1.0 \ 1.1 aren't in support any more, so we might be able to remove these tests without figuring out why they're failing so.

[JunTaoLuo]

cc @dougbu for review.