Moving Sdk import of standard libraries into targets does not work

I tried the following
My project.csproj:
```xml

```
The Test.targets file:
```xml

netstandard2.0

```
Sample:
[NetStandardLib.zip](https://github.com/Microsoft/msbuild/files/1272817/NetStandardLib.zip)

Expected behavior:
I can see all cs files in Visual Studio as if it would be when i define the sdk directly within the project file
I can see the SDK Node under the project tree node in VS
The build is successful

Actual behavior:
No cs files are displayed, but they are built
I can see all implicit referenced standard libaries with their full path under the node Project\references
The build fails after reopening the project with many errors

Why i want to do that?
I have already a target framework that allows me to extract all common things (e.g. Configurations) between different types of projects (e.g. .net, android, pcl). Actually its a solution that is quite similar to the new standard library project format.

To update the version of netstandard libary for many projects, i want to be able to do that without touching the projects

I really like the new slim format of standard libararies, but it seems that it breaks with many MSBuild features. In big project environments that could lead to frustration because you can't rely on MSBuild features that u could use without any problems in the past.