-e: see title
--workaround: delete Library folder and rebuild (doesn't always work it seems)

-repro:
--create empty project in 5.6
--make a WebGL build with the default settings
--notice it succeeds
--open project with 2017.1b1
--switch to .NET 4.6
--start WebGL build
--NOTICE build fails with errors
---IL2CPP error for type 'Microsoft.Win32.SafeHandles.SafeHandleZeroOrMinusOneIsInvalid' in assembly 'D:\Work\Bugs\New Unity Project 21\Temp\StagingArea\Data\Managed\mscorlib.dll'
Additional information: Build a development build for more information. Sequence contains no matching element