title: Can not find runtime target for framework .NETCoreApp=v1 compatible with one of the target runtimes for some projects
body: <p>I have a solution with many .NET Core projects. I did NuGet updates to all of the project and now when I try to build I get the following errors (for some of the projects - not all):</p>
  
  <pre><code>Can not find runtime target for framework '.NETCoreApp,Version=v1.0' compatible with one of the target runtimes: 'win10-x64, win81-x64, win8-x64, win7-x64'. Possible causes:
  1. The project has not been restored or restore failed - run `dotnet restore`
  2. The project does not list one of 'win10-x64, win81-x64, win8-x64, win7-x64' in the 'runtimes' section.
  3. You may be trying to publish a library, which is not supported. Use `dotnet pack` to distribute libraries.
  </code></pre>
  
  <p>What seemed to help was a post I found here <a href="https://stackoverflow.com/questions/37590604/can-not-find-runtime-target-for-framework-netcoreapp-v1-compatible-with-one-of">Can not find runtime target for framework .NETCoreApp=v1 compatible with one of the target runtimes</a></p>
  
  <p>I added the following to the failing projects:</p>
  
  <pre><code>"runtimes": {
     "win10-x64": { }
  }
  </code></pre>
  
  <p>That seemed to fix the issue. But my question is, why was this happening only on some projects? The projects that did not issue the error doesn't have such runtime definition in their <code>project.json</code> file.</p>
  
  <p>What exactly does this runtime definition means? how does it affect my ability to run on other os like linux or mac?</p>