title: Missing System.Core.dll and others on Visual Studio 2010 Project targeting 3.5 Framework
body: <p>I just got Visual Studio 2010 installed and running on my development machine (alongside VS 2008). The first thing I did was make a copy of an existing project and convert it up to the VS 2010 project. I told the conversion wizard to leave the project targeting the 3.5 framework. The project compiles and runs just fine in VS 2008. When I go to build it, several of my references are marked as missing (System.Core, System.Xml.Linq, System.Data.DataSetExtensions). When I go to the Add Reference dialog, they are not present. Yet if I view the GAC at C:\Windows\Assembly, I can see the entries there.</p>
  
  <p>I have tried converting the projects to 4.0, then back down to 3.5, and also unloading/reloading the projects. Nothing works. Has anyone else ran across this problem?</p>