Test.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>netstandard1.4</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <None Include="Foo1.cs" Condition="'$(defaultTrue)' == 'true'" />
    <!--#if (defaultTrue) -->
    <None Include="Foo2.cs" />
    <!--#endif -->

    <None Include="Foo3.cs" Condition="'$(defaultFalse)' == 'true'" />
    <!--#if (defaultFalse) -->
    <None Include="Foo4.cs" />
    <!--#endif -->

<!--/-:msbuild-conditional:noEmit -->
    <None Include="Foo5.cs" Condition="'$(defaultFalse)' == 'true'" />
<!--/+:msbuild-conditional:noEmit -->
    <None Include="Foo5.1.cs" Condition="'$(defaultTrue)' == 'true'" />

<!-- Without noEmit the following line will be emitted -->
<!--/-:msbuild-conditional -->
    <None Include="Foo5.2.cs" Condition="'$(defaultFalse)' == 'true'" />
<!--/+:msbuild-conditional -->
    <None Include="Foo5.3.cs" Condition="'$(defaultTrue)' == 'true'" />

<!--/-:cnd:noEmit -->
    <!--#if (defaultFalse) -->
    <None Include="Foo6.cs" />
    <!--#endif -->
<!--/+:cnd:noEmit -->
    <!--#if (defaultTrue) -->
    <None Include="Foo6.6.cs" />
    <!--#endif -->

    <None Include="Things" />
  </ItemGroup>
</Project>