sdk: dotnet sln add returns 'Project 'foo.csproj' has an unknown project type and cannot be added to the solution file. Please contact your SDK provider for support.'

Steps to reproduce

1. Create a new SDK project (dotnet new console is fine)
2. Change the csproj so that it targets more than one framework (<TargetFrameworks>netcoreapp2.0;net461</TargetFrameworks>, for example)
3. Create a new solution using the CLI (dotnet new sln)
4. Try to add the project from step 1 to the solution created in step 3 (dotnew sln <sln> add <csproj>)

Expected behavior

• Project 'foo.csproj' added to the solution.
• Project is added to the solution

Actual behavior

• Project 'foo.csproj' has an unknown project type and cannot be added to the solution file. Please contact your SDK provider for support.
• Project is not added to the solution

More details

dotnet build on the project just works, so it doesn’t look like an issue with targeting packs.

Environment data

dotnet --info output:

.NET Core SDK (reflecting any global.json):
 Version:   2.1.300
 Commit:    adab45bf0c

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.17134
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\2.1.300\

Host (useful for support):
  Version: 2.1.0
  Commit:  caa7b7e2ba

.NET Core SDKs installed:
  1.1.9 [C:\Program Files\dotnet\sdk]
  2.1.201 [C:\Program Files\dotnet\sdk]
  2.1.300 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 1.0.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 1.1.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.0.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]