Use default item metadata so that this works in C# as well.

src/Compilers/VisualBasic/Portable/BasicCodeAnalysis.vbproj

@@ -970,9 +970,7 @@
     <Compile Include="Compilation\ModuleCompilationState.vb" />
     <PublicAPI Include="PublicAPI.txt" />
     <Content Include="Symbols\SymbolsAndNoPia.docx" />
-    <ErrorCode Include="Errors\Errors.vb">
-      <Visible>false</Visible>
-    </ErrorCode>
+    <ErrorCode Include="Errors\Errors.vb" />
     <SyntaxDefinition Include="Syntax\Syntax.xml">
       <SubType>Designer</SubType>
     </SyntaxDefinition>
@@ -1029,4 +1027,4 @@
     <Import Project="..\..\..\..\build\VSL.Imports.Closed.targets" />
     <Import Project="$(SolutionDir)\.nuget\NuGet.targets" Condition="Exists('$(SolutionDir)\.nuget\NuGet.targets')" />
   </ImportGroup>
-</Project>
\ No newline at end of file
+</Project>

src/Tools/Microsoft.CodeAnalysis.Toolset.Open/Targets/GenerateCompilerInternals.targets

@@ -171,6 +171,12 @@
     <Delete Files="$(GeneratedBoundTree)" />
   </Target>
 
+  <ItemDefinitionGroup>
+    <ErrorCode>
+      <Visible>false</Visible>
+    </ErrorCode>
+  </ItemDefinitionGroup>
+
   <Target
     Name="GenerateErrorFacts"
     Inputs="@(ErrorCode);$(CSharpErrorFactsGeneratorToolPath);$(VBErrorFactsGeneratorToolPath)"