Address feedback from #79828. (#80509)

Author: eiriktsarpalis

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.TestLibrary/System.Text.Json.TestLibrary.Roslyn4.4.csproj

@@ -3,6 +3,8 @@
   <Import Project="System.Text.Json.TestLibrary.targets" />
 
   <ItemGroup>
-    <ProjectReference Include="..\..\gen\System.Text.Json.SourceGeneration.Roslyn4.4.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" />
+    <!-- The Roslyn4.4 project uses multi-targeting to enable nullability annotations,
+         so we need to explicitly reference the ns2.0 TFM used by the production source generator -->
+    <ProjectReference Include="..\..\gen\System.Text.Json.SourceGeneration.Roslyn4.4.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" SetTargetFramework="TargetFramework=netstandard2.0" />
   </ItemGroup>
 </Project>

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/System.Text.Json.SourceGeneration.Roslyn4.4.Tests.csproj

@@ -12,7 +12,9 @@
   <Import Project="System.Text.Json.SourceGeneration.Tests.targets" />
 
   <ItemGroup>
-    <ProjectReference Include="..\..\gen\System.Text.Json.SourceGeneration.Roslyn4.4.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" SetTargetFramework="TargetFramework=netstandard2.0" SkipGetTargetFrameworkProperties="true" />
+    <!-- The Roslyn4.4 project uses multi-targeting to enable nullability annotations,
+         so we need to explicitly reference the ns2.0 TFM used by the production source generator -->
+    <ProjectReference Include="..\..\gen\System.Text.Json.SourceGeneration.Roslyn4.4.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" SetTargetFramework="TargetFramework=netstandard2.0" />
     <ProjectReference Include="..\System.Text.Json.SourceGeneration.TestLibrary\System.Text.Json.TestLibrary.Roslyn4.4.csproj" />
   </ItemGroup>
 </Project>

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/System.Text.Json.SourceGeneration.Roslyn4.4.Unit.Tests.csproj

@@ -12,6 +12,8 @@
   <Import Project="System.Text.Json.SourceGeneration.Unit.Tests.targets" />
 
   <ItemGroup>
-    <ProjectReference Include="..\..\gen\System.Text.Json.SourceGeneration.Roslyn4.4.csproj" />
+    <!-- The Roslyn4.4 project uses multi-targeting to enable nullability annotations,
+         so we need to explicitly reference the ns2.0 TFM used by the production source generator -->
+    <ProjectReference Include="..\..\gen\System.Text.Json.SourceGeneration.Roslyn4.4.csproj" SetTargetFramework="TargetFramework=netstandard2.0" />
   </ItemGroup>
 </Project>