Fix references to socket APIs, and target the lowest net standard versions possible

src/Microsoft.AspNetCore.NodeServices/HostingModels/PhysicalConnections/NamedPipeConnection.cs

@@ -9,12 +9,18 @@ namespace Microsoft.AspNetCore.NodeServices.HostingModels.PhysicalConnections
         private bool _disposedValue = false;
         private NamedPipeClientStream _namedPipeClientStream;
 
+#pragma warning disable 1998 // Because in the NET451 code path, there's nothing to await
         public override async Task<Stream> Open(string address)
         {
             _namedPipeClientStream = new NamedPipeClientStream(".", address, PipeDirection.InOut);
+#if NET451
+            _namedPipeClientStream.Connect();
+#else
             await _namedPipeClientStream.ConnectAsync().ConfigureAwait(false);
+#endif
             return _namedPipeClientStream;
         }
+#pragma warning restore 1998
 
         public override void Dispose()
         {

src/Microsoft.AspNetCore.NodeServices/HostingModels/PhysicalConnections/StreamConnection.cs

@@ -1,6 +1,5 @@
 using System;
 using System.IO;
-using System.Runtime.InteropServices;
 using System.Threading.Tasks;
 
 namespace Microsoft.AspNetCore.NodeServices.HostingModels.PhysicalConnections
@@ -12,7 +11,11 @@ namespace Microsoft.AspNetCore.NodeServices.HostingModels.PhysicalConnections
 
         public static StreamConnection Create()
         {
-            var useNamedPipes = RuntimeInformation.IsOSPlatform(OSPlatform.Windows);
+#if NET451
+            return new NamedPipeConnection();
+#else
+            var useNamedPipes = System.Runtime.InteropServices.RuntimeInformation.IsOSPlatform(
+                System.Runtime.InteropServices.OSPlatform.Windows);
             if (useNamedPipes)
             {
                 return new NamedPipeConnection();
@@ -21,6 +24,7 @@ namespace Microsoft.AspNetCore.NodeServices.HostingModels.PhysicalConnections
             {
                 return new UnixDomainSocketConnection();
             }
+#endif
         }
     }
 }

src/Microsoft.AspNetCore.NodeServices/HostingModels/PhysicalConnections/UnixDomainSocketConnection.cs

@@ -10,6 +10,16 @@ namespace Microsoft.AspNetCore.NodeServices.HostingModels.PhysicalConnections
         private NetworkStream _networkStream;
         private Socket _socket;
 
+#if NET451
+        public override Task<Stream> Open(string address)
+        {
+            // The 'null' assignments avoid the compiler warnings about unassigned fields.
+            // Note that this whole class isn't supported on .NET 4.5.1, since that's not cross-platform.
+            _networkStream = null;
+            _socket = null;
+            throw new System.PlatformNotSupportedException();
+        }
+#else
         public override async Task<Stream> Open(string address)
         {
             var endPoint = new UnixDomainSocketEndPoint("/tmp/" + address);
@@ -18,6 +28,7 @@ namespace Microsoft.AspNetCore.NodeServices.HostingModels.PhysicalConnections
             _networkStream = new NetworkStream(_socket);
             return _networkStream;
         }
+#endif
 
         public override void Dispose()
         {

src/Microsoft.AspNetCore.NodeServices/HostingModels/PhysicalConnections/UnixDomainSocketEndPoint.cs

@@ -59,7 +59,11 @@ namespace Microsoft.AspNetCore.NodeServices.HostingModels.PhysicalConnections
             }
             else
             {
+#if NET451
+                _encodedPath = new byte[0];
+#else
                 _encodedPath = Array.Empty<byte>();
+#endif
                 _path = string.Empty;
             }
         }

src/Microsoft.AspNetCore.NodeServices/HostingModels/VirtualConnections/VirtualConnection.cs

@@ -12,6 +12,11 @@ namespace Microsoft.AspNetCore.NodeServices.HostingModels.VirtualConnections
     /// </summary>
     internal class VirtualConnection : Stream
     {
+#if NET451
+        private readonly static Task CompletedTask = Task.FromResult((object)null);
+#else
+        private readonly static Task CompletedTask = Task.CompletedTask;
+#endif
         private VirtualConnectionClient _host;
         private readonly BufferBlock<byte[]> _receivedDataQueue = new BufferBlock<byte[]>();
         private ArraySegment<byte> _receivedDataNotYetUsed;
@@ -109,7 +114,7 @@ namespace Microsoft.AspNetCore.NodeServices.HostingModels.VirtualConnections
                 throw new InvalidOperationException("The connection was already closed by the remote party");
             }
 
-            return count > 0 ? _host.WriteAsync(Id, buffer, offset, count, cancellationToken) : Task.CompletedTask;
+            return count > 0 ? _host.WriteAsync(Id, buffer, offset, count, cancellationToken) : CompletedTask;
         }
 
         public override int Read(byte[] buffer, int offset, int count)