fix Grpc.Tools build

src/csharp/Grpc.Tools.Tests/Grpc.Tools.Tests.csproj

@@ -7,10 +7,14 @@
 
   <Import Project="..\Grpc.Core\SourceLink.csproj.include" />
 
-  <!-- Needed for the net45 build to work on Unix. See https://github.com/dotnet/designs/pull/33 -->
-  <ItemGroup>
-    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0-preview.2" PrivateAssets="All" />
-  </ItemGroup>
+  <PropertyGroup Condition=" '$(OS)' != 'Windows_NT' and '$(MSBuildRuntimeType)' == 'Core' ">
+    <!-- Use Mono reference assemblies in SDK build: https://github.com/dotnet/sdk/issues/335.
+         This is a different approach than used in Grpc.Core/Common.csproj.include because
+         the workaround used there doesn't seem to be working for Microsoft.Build.* assemblies -->
+    <FrameworkPathOverride Condition="Exists('/usr/lib/mono/4.5-api')">/usr/lib/mono/4.5-api</FrameworkPathOverride>
+    <FrameworkPathOverride Condition="Exists('/usr/local/lib/mono/4.5-api')">/usr/local/lib/mono/4.5-api</FrameworkPathOverride>
+    <FrameworkPathOverride Condition="Exists('/Library/Frameworks/Mono.framework/Versions/Current/lib/mono/4.5-api')">/Library/Frameworks/Mono.framework/Versions/Current/lib/mono/4.5-api</FrameworkPathOverride>
+  </PropertyGroup>
 
   <ItemGroup>
     <ProjectReference Include="..\Grpc.Tools\Grpc.Tools.csproj" />

src/csharp/Grpc.Tools/Grpc.Tools.csproj

@@ -7,10 +7,14 @@
     <TargetFrameworks>net45;netstandard1.3</TargetFrameworks>
   </PropertyGroup>
 
-  <!-- Needed for the net45 build to work on Unix. See https://github.com/dotnet/designs/pull/33 -->
-  <ItemGroup>
-    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0-preview.2" PrivateAssets="All" />
-  </ItemGroup>
+  <PropertyGroup Condition=" '$(OS)' != 'Windows_NT' and '$(MSBuildRuntimeType)' == 'Core' ">
+    <!-- Use Mono reference assemblies in SDK build: https://github.com/dotnet/sdk/issues/335.
+         This is a different approach than used in Grpc.Core/Common.csproj.include because
+         the workaround used there doesn't seem to be working for Microsoft.Build.* assemblies -->
+    <FrameworkPathOverride Condition="Exists('/usr/lib/mono/4.5-api')">/usr/lib/mono/4.5-api</FrameworkPathOverride>
+    <FrameworkPathOverride Condition="Exists('/usr/local/lib/mono/4.5-api')">/usr/local/lib/mono/4.5-api</FrameworkPathOverride>
+    <FrameworkPathOverride Condition="Exists('/Library/Frameworks/Mono.framework/Versions/Current/lib/mono/4.5-api')">/Library/Frameworks/Mono.framework/Versions/Current/lib/mono/4.5-api</FrameworkPathOverride>
+  </PropertyGroup>
 
   <PropertyGroup Label="Asset root folders. TODO(kkm): Change with package separation.">
     <!-- TODO(kkm): Rework whole section when splitting packages.  -->