title: Upgrading to .NET Core 2.0: PackageTargetFallback and AssetTargetFallback cannot be used together
body: <p>When upgrading a project that is compiling and running fine on .NET Core 1.0 or 1.1, updating the target framework to .NET Core 2.0 (<code>netcoreapp2.0</code>) causes the build to fail with the following error:</p>
  
  <pre><code> error NU1003: PackageTargetFallback and AssetTargetFallback cannot be used together. Remove PackageTargetFallback(deprecated) references from the project environment.
  </code></pre>
  
  <p>What is the reason for this error and how can it be resolved to make the project build again?</p>