To do this, you'll need to change the .csproj manually, and add a Condition XML Attribute in the Compile XML element corresponding to the file you want only in debug builds. Like this:

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="3.5" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
      ...
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
      ...
  </PropertyGroup>
  <ItemGroup>
    <!-- Add the Condition attribute here, on each of your debug only files
    Pick up one of the configuration definition configuration above -->
    <Compile Include="DebugOnlyClass1.cs" Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' "/>
    <!-- normal compilation -->
    <Compile Include="Class1.cs" />
    <Compile Include="Properties\AssemblyInfo.cs" />
  </ItemGroup>
     ...
</Project>