title: Identity value generation can only be used with signed integer properties after upgrade to EF Core 1.1
body: <p>When I updated <strong>"Microsoft.EntityFrameworkCore.Tools.DotNet"</strong> to version  <strong>"1.1.0-preview4"</strong>, entity framework has stopped to generate migrations.</p>
  
  <p>Error: </p>
  
  <pre><code>dotnet : System.ArgumentException: Identity value generation cannot be used      
  for the property 'UID' on entity type 'SomeEntity' because the property type is   
  'Guid'. Identity value generation can only be used with signed integer properties.
  </code></pre>