Lib: support ImplicitUsings

Author: dahlbyk

src/PolyfillLib/PolyfillLib.csproj

@@ -17,4 +17,7 @@
   </ItemGroup>
   <Import Project="$(SolutionDir)\TestIncludes.targets" />
   <Import Project="$(SolutionDir)\Polyfill\Polyfill.targets" />
+  <ItemGroup Condition="$(Language) == 'C#' and ($(ImplicitUsings) == 'enable' or $(ImplicitUsings) == 'true')">
+    <Using Include="Polyfills" />
+  </ItemGroup>
 </Project>