Use $(TargetFramework) in the SDK when only targeting a single TFM

[eerhardt]

There were 2 places I didn't change that should be.

1. I couldn't update https://github.com/dotnet/sdk/blob/master/TestAssets/TestProjects/CompilationContext/TestLibrary/TestLibrary.csproj because of Having a TestApp with $(TargetFrameworks) P2P reference a TestLibrary with $(TargetFramework) doesn't build #255.
2. I didn't update https://github.com/dotnet/sdk/tree/master/src/Templates/ProjectTemplates/CSharp/Web/EmptyWeb because of Web projects not working with cross-targeting #210.

@nguerrera @srivatsn

[eerhardt]

/cc @dotnet/project-system @piotrpMSFT

[srivatsn]

@mlorbetske @abpiskunov @phenning we should update all the web templates to use TargetFramework instead of TargetFrameworks as well if they have just one TF.

[nguerrera]

nit: whitespace

[eerhardt]

oops.

[nguerrera]

Is something setting Configuration env var in the SDK build too? I see we have SkipInvalidConfigurations in restore command and that's still just downgrading ~20 errors to warnings. I think that:

1. we also need to force /p:_InvalidConfigurationWarning=false to eliminate the warnings.
2. we should get the nuget restore target suppress these itself.

[nguerrera]

I didn't update https://github.com/dotnet/sdk/tree/master/src/Templates/ProjectTemplates/CSharp/Web/EmptyWeb because of #210.

I think it's fine to update it to set single target framework after props since that was because of the nuget bug. We still need to verify if multiple targetframeworks works with web, but this doesn't impact the template any longer since templates are supposed to be single target framework now and with the nuget bug fixed, this can be done in the right place.

[eerhardt]

Is something setting Configuration env var in the SDK build too?

We pass the Configuration into our call into MSBuild: https://github.com/dotnet/sdk/blob/master/build.ps1#L62

We need to fix the bug: #203

[nguerrera]

We need to fix the bug: #203

Agreed. Until then and as long as we're having to work around it with /p:SkipInvalidConfigurations=true, can we also pass /p:_InvalidConfigurationWarning=false. Or can we not pass configuration to the restore invocations? I find it really distracting for every build to have a sea of yellow and more critically, it lets new warnings slip in unnoticed.

[nguerrera]