Use MSBuild to set NuGet feeds instead of NuGet.config

Directory.Build.targets

@@ -1,4 +1,5 @@
 <Project>
+  <Import Project="build\sources.props" />
   <Import Project="build\dependencies.props" />
   <Import Project="build\dependencies.targets" />
   <Import Project="build\dependencies.g.targets" Condition="Exists('build\dependencies.g.targets') AND '$(DesignTimeBuild)' != 'true'" />

NuGet.config

@@ -2,9 +2,6 @@
 <configuration>
   <packageSources>
     <clear />
-    <add key="AspNetCorePatch" value="https://dotnet.myget.org/F/aspnet-2-0-2-october2017-patch/api/v3/index.json" />
-    <add key="AspNetCore" value="https://dotnet.myget.org/F/aspnetcore-master/api/v3/index.json" />
-    <add key="AspNetCoreTools" value="https://dotnet.myget.org/F/aspnetcore-tools/api/v3/index.json" />
-    <add key="NuGet" value="https://api.nuget.org/v3/index.json" />
+    <!-- Restore sources should be defined in build/sources.props. -->
   </packageSources>
 </configuration>

build/sources.props

@@ -0,0 +1,14 @@
+<Project>
+  <Import Project="$(DotNetRestoreSourcePropsPath)" Condition="'$(DotNetRestoreSourcePropsPath)' != ''"/>
+
+  <PropertyGroup>
+    <RestoreSources>$(DotNetRestoreSources)</RestoreSources>
+    <RestoreSources Condition="'$(DotNetBuildOffline)' != 'true'">
+      $(RestoreSources);
+      https://dotnet.myget.org/F/aspnet-2-0-2-october2017-patch/api/v3/index.json;
+      https://dotnet.myget.org/F/aspnetcore-master/api/v3/index.json;
+      https://dotnet.myget.org/F/aspnetcore-tools/api/v3/index.json;
+      https://api.nuget.org/v3/index.json;
+    </RestoreSources>
+  </PropertyGroup>
+</Project>