NuGet error restoring packages
<p>Suddenly today I click Rebuild on my sln and get...</p>

<p>Error (~10 similar errors):</p>

<blockquote>
  <p>Severity  Code    Description Project File    Line    Suppression State
  Error     NuGet Package restore failed for project Core: Unable to find
  version '4.0.11' of package 'System.Diagnostics.Debug'..          0</p>
</blockquote>

<p><a href="https://www.nuget.org/packages/System.Diagnostics.Debug/" rel="nofollow">System.Diagnostics.Debug 4.0.11</a> exists.</p>

<p><strong>Nuget version:</strong>  3.3.0.212</p>

<p>Other stuff:</p>

<ul>
<li>At PM console:  Install-Package System.Diagnostics.Debug -Version 4.0.11</li>
</ul>

<p>Error: </p>

<blockquote>
  <p>Install-Package : Some NuGet packages are missing from the solution.
  The packages need to be restored in order to build the dependency 
  graph. Restore the packages before performing any operations.</p>
</blockquote>

<ul>
<li><code>&lt;RestorePackages&gt;</code> is not <a href="http://docs.nuget.org/consume/package-restore/migrating-to-automatic-package-restore" rel="nofollow">listed</a> in the .sln.  So I don't think this thing is configured the <a href="http://blog.davidebbo.com/2014/01/the-right-way-to-restore-nuget-packages.html" rel="nofollow">old way</a>.</li>
</ul>