To prevent the build from also copying the Roslyn files to the bin directory, you have to also comment out this line that gets placed at the top of your Web application project:
<!-- <Import Project="..\..\..\DAS\packages\Microsoft.CodeDom.Providers.DotNetCompilerPlatform.1.0.1\build\Microsoft.CodeDom.Providers.DotNetCompilerPlatform.props" Condition="Exists('..\..\..\DAS\packages\Microsoft.CodeDom.Providers.DotNetCompilerPlatform.1.0.1\build\Microsoft.CodeDom.Providers.DotNetCompilerPlatform.props')" /> -->