Adjust TranscodingStream and related code to compile and run here

- add conditions to avoid references to version-specific API
- remove references to internal dotnet/runtime code
  - rewrite some code more generically if performance impact likely minor
- use Microsoft.TestCommon instead of directly relying on Xunit in tests
- use C# 9.0

Author: dougbu

src/System.Net.Http.Formatting.NetCore/System.Net.Http.Formatting.NetCore.csproj

@@ -18,8 +18,10 @@
     <PackageReference Include="System.Collections.Specialized" Version="4.3.0" />
     <PackageReference Include="System.ComponentModel.EventBasedAsync" Version="4.3.0" />
     <PackageReference Include="System.Diagnostics.Contracts" Version="4.3.0" />
+    <PackageReference Include="System.Memory" Version="4.5.5" />
     <PackageReference Include="System.Runtime.Serialization.Json" Version="4.3.0" />
     <PackageReference Include="System.Runtime.Serialization.Xml" Version="4.3.0" />
+    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.4" />
     <PackageReference Include="System.Xml.XmlSerializer" Version="4.3.0" />
 
     <PackageReference Include="Newtonsoft.Json" Version="13.0.1" />

src/System.Net.Http.Formatting.NetStandard/System.Net.Http.Formatting.NetStandard.csproj

@@ -15,6 +15,9 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="System.Memory" Version="4.5.5" />
+    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.4" />
+
     <PackageReference Include="Newtonsoft.Json" Version="13.0.1" />
     <PackageReference Include="Newtonsoft.Json.Bson" Version="1.0.2" />
 

src/System.Net.Http.Formatting/Internal/NullableAttributes.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+// From https://github.com/dotnet/runtime/blob/88868b7a781f4e5b9037b8721f30440207a7aa42/src/tools/illink/src/ILLink.RoslynAnalyzer/NullableAttributes.cs
+
 namespace System.Diagnostics.CodeAnalysis
 {
 #if !NETSTANDARD2_1