title: Package [some package] is not compatible with netcoreapp
body: <p>I recently installed Visual Studio 2017. I migrated the existing project which created in Visual Studio 2015. It got migrated without any errors. But when I build the solution I get 100+ errors stating <code>Package [Microsoft.AspNetCore...] is not compatible with [netcoreapp...]</code></p>
  
  <p>There are more than 100 items listed like this. I tried 
  renaming the <code>sdk</code> value in <code>global.json</code> 
  removing <code>sdk</code> value in <code>global.json</code>
  repair, uninstall &amp; reinstalling Visual Studio 2017. But none of them work.</p>
  
  <p>I'm not sure where to add <code>net451</code> in <code>imports</code> as explained <a href="https://github.com/reactjs/React.NET/issues/275" rel="nofollow noreferrer">here</a> because the <code>project.json</code> file is deleted while migration.</p>
  
  <p>The same project was working fine in Visual Studio 2015. All issues started after migrating to Visual Studio 2017.</p>
  
  <p>Please let me know how to fix this <a href="https://i.stack.imgur.com/EVNYW.png" rel="nofollow noreferrer">error</a>.</p>