Open a PowerShell prompt with elevated privilege (as Administrator)

Get nuget.exe command line wget https://dist.nuget.org/win-x86-commandline/latest/nuget.exe -OutFile nuget.exe

Download the C# Roslyn compiler (just a few megs, no need to 'install') .nuget.exe install Microsoft.Net.Compilers

Note: as of date 21 Aug 2018, this installs the Roslyn compiler here:

C:WindowsSystem32Microsoft.Net.Compilers.2.9.0tools