title: Upgrading System.Collections.Immutable to 1.2 breaks application
body: <p>I have a working Code Analysis VSIX that used version 1.1.37 and I upgrade the NuGet to the latest 1.2 because many other NuGet libraries I want to use require the latest version. When I run my application with no code changes I get an error that the older version can't be found. This is the only library I have ever had trouble upgrading. If I roll back to older version application works. </p>
  
  <p>I am running VB under Visusl Studio 2015 Update 3 which came with the older 1.1.37 version. Unfortunately if I try to set a break in my code I never got to it the error happens somewhere in Roslyn or System code before my code gets called. </p>