Build with CodeContracts fails with dotnet build

[johncrim]

By manually adding imports and msbuild properties (b/c no IDE support in VS 2017), ccrewrite works on netcoreapp and netstandard projects (only when portable PDBs are not used).

Running the build using msbuild /t:build works. Running the build using dotnet build fails:

dotnet build /v:normal
Build started 2017-09-25 9:06:34 AM
...
       CreateCodeContractReferenceAssembly:
         "C:\Program Files (x86)\Microsoft\Contracts\Bin\ccrefgen.exe" "@obj\Debug\netstandard2.0\MyProject.ccrefgen.rsp"
         elapsed time: 881.9998ms
       CodeContractRewrite:
         "C:\Program Files (x86)\Microsoft\Contracts\Bin\ccrewrite.exe" "@MyProject.ccrewrite.rsp"
                There were errors reported in Tv.Web.Upfront.Model's metadata.
                Exception of type 'System.OutOfMemoryException' was thrown.
     1>ccrewrite : error : Foxtrot:     There were errors reported in Tv.Web.Upfront.Model's metadata. [C:\src\my\project\MyProject.csproj]
                Exception of type 'System.OutOfMemoryException' was thrown.
         elapsed time: 61.4774ms
     1>C:\Program Files (x86)\Microsoft\Contracts\MsBuild\v15.0\Microsoft.CodeContracts.targets(275,5): error MSB3073: The command ""C:\Program Files (x86)\Microsoft\Contracts\Bin\ccrewrite.exe" "@MyProject.ccrewrite.rsp"" exited with code -1. [C:\src\my\project\MyProject.csproj]
     1>Done Building Project "C:\src\my\project\MyProject.csproj" (Build target(s)) -- FAILED.

Build FAILED.

The root cause is this block of the Microsoft.CodeContracts.targets file:
https://github.com/Microsoft/CodeContracts/blob/master/Microsoft.Research/ManagedContract.Setup/MSBuild/v15.0/Microsoft.CodeContracts.targets#L292

Target GetFrameworkSdkPath, and sn.exe don't exist in the dotnet build system.