Fix build warning and treat msbuild warnings as errors

TestAssets/TestPackages/dotnet-dependency-tool-invoker/dotnet-dependency-tool-invoker.csproj

@@ -1,8 +1,9 @@
-<Project Sdk="Microsoft.NET.Sdk" ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+<Project>
 
   <!-- This test asset needs to import the general dir.props in order to get the SdkNugetVersion property.
       This is why it also needs to explicitly set DisableImplicitFrameworkReferences to false. -->
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.props))\dir.props" />
+  <Import Sdk="Microsoft.NET.Sdk" Project="Sdk.props" />
 
   <PropertyGroup>
     <VersionPrefix>1.0.0-rc</VersionPrefix>
@@ -23,4 +24,5 @@
     <PackageReference Include="System.Linq" Version="4.3.0" />
   </ItemGroup>
 
+  <Import Sdk="Microsoft.NET.Sdk" Project="Sdk.targets" />
 </Project>

build/BuildDefaults.props

@@ -33,5 +33,6 @@
     <NoWarn>NU1701;NU5104</NoWarn>
 
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <MSBuildTreatWarningsAsErrors>true</MSBuildTreatWarningsAsErrors>
   </PropertyGroup>
 </Project>