No, `System.Runtime.dll` is a PCLv2 Facade Assembly which only exists in this version.

We could actually just check for the short name `System.Runtime`, that's that MSBuild does (search for `_HasReferenceToSystemRuntime` in `Microsoft.NETFramework.targets`).  However, I thought using the full name is cleaner and I'm also doing that in our xbuild task.