Issue
Be aware that the NuGet PM breaks the Rosalyn behavior. Click Tools > NuGet Package Manager > Manage NuGet Packages for Solution If an update Exists for Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Microsoft.Net.Compilers, or Microsoft.Net.Compilers.netcore, update them and the Solution will break! This occurs because the ASP Sites templates are set to use specific versions at project creation. To See the problem, click Show All Files in the Solution Explorer.
Fix
At project creation the $(WebProjectOutputDir)\bin doesn't exist, therefore when Rosalyn is added as a dependency by NuGet it installs it properly. After Updating the Solution Packages, the $(WebProjectOutputDir)\bin directory looks like so:
$(WebProjectOutputDir)\bin\bin\rosalyn
The easiest fix is to Cut & Paste rosalyn to the proper location, and then delete the extra bin folder. You can now Refresh the page and the site will load.