It's not necessary to specify an exact System.Collections.Concurrent dependency, since we can consume it through NETStandard.Library

nblumhardt · nblumhardt · commit 409e4fd8a861 · 2018-05-18T19:40:31.000+10:00
diff --git a/src/Serilog.Sinks.Async/Serilog.Sinks.Async.csproj b/src/Serilog.Sinks.Async/Serilog.Sinks.Async.csproj
@@ -35,8 +35,4 @@
     <Reference Include="Microsoft.CSharp" />
   </ItemGroup>
 
-  <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard1.1' ">
-    <PackageReference Include="System.Collections.Concurrent" Version="4.3.0" />
-  </ItemGroup>
-
 </Project>
