Let me first start by saying, I'm not even sure what I'm trying to do is possible, and I'm very new to Xamarin/Monotouch development, but here goes.
For some background, I'm looking for a Xamarin PCL version of a bi-directional Pusher library. I've found an implementation that receives, but doesn't send. I've used the Pusher.net library before in a Windows app, and it worked well, so I figured I'd try to port it. I'm able to import and compile the actual Pusher assembly as a PCL with with no problems. However when I try to build the Pusher.Connections.Net library (again, as a PCL), I get the following error:
Target CoreCompile:
Tool /Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/4.5/mcs.exe execution started with arguments: /noconfig /debug:full /debug+ /optimize- /out:obj/Debug/Pusher.Connections.dll Properties/AssemblyInfo.cs WebSocketConnection.cs WebSocketConnectionFactory.cs /target:library /define:DEBUG /nostdlib /reference:../../../Downloads/WebSocket4Net(0.11)/net45/Debug/WebSocket4Net.dll /reference:../packages/Xamarin.Forms.1.3.1.6296/lib/portable-win+net45+wp80+MonoAndroid10+MonoTouch10+Xamarin.iOS10/Xamarin.Forms.Core.dll /reference:../packages/Xamarin.Forms.1.3.1.6296/lib/portable-win+net45+wp80+MonoAndroid10+MonoTouch10+Xamarin.iOS10/Xamarin.Forms.Xaml.dll /reference:/Users/user/Projects/PusherTest/Pusher/bin/Debug//Pusher.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/Microsoft.CSharp.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Collections.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.ComponentModel.EventBasedAsync.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.ComponentModel.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Core.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Diagnostics.Contracts.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Diagnostics.Debug.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Diagnostics.Tools.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Dynamic.Runtime.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Globalization.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.IO.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Linq.Expressions.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Linq.Queryable.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Linq.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Net.NetworkInformation.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Net.Primitives.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Net.Requests.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Net.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.ObjectModel.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Reflection.Extensions.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Reflection.Primitives.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Reflection.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Resources.ResourceManager.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Runtime.Extensions.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Runtime.InteropServices.WindowsRuntime.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Runtime.Serialization.Json.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Runtime.Serialization.Primitives.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Runtime.Serialization.Xml.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Runtime.Serialization.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Runtime.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Security.Principal.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.ServiceModel.Http.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.ServiceModel.Primitives.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.ServiceModel.Security.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.ServiceModel.Web.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.ServiceModel.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Text.Encoding.Extensions.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Text.Encoding.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Text.RegularExpressions.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Threading.Tasks.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Threading.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Windows.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Xml.Linq.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Xml.ReaderWriter.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Xml.Serialization.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Xml.XDocument.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Xml.XmlSerializer.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.Xml.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/System.dll /reference:/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild-frameworks/.NETPortable/v4.5/Profile/Profile78/mscorlib.dll /warn:4
WebSocketConnection.cs(94,27): warning CS1998: Async block lacks `await' operator and will run synchronously
WebSocketConnection.cs(24,14): error CS1684: Reference to type `System.Net.EndPoint' claims it is defined assembly `System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089', but it could not be found
/Users/user/Projects/PusherTest/Pusher.Connections/../../../Downloads/WebSocket4Net(0.11)/net45/Debug/WebSocket4Net.dll (Location of the symbol related to previous error)
Task "Csc" execution -- FAILED
Done building target "CoreCompile" in project "/Users/user/Projects/PusherTest/Pusher.Connections/Pusher.Connections.Net.csproj".-- FAILED
To save some scrolling, the key error is:
WebSocketConnection.cs(24,14): error CS1684: Reference to type `System.Net.EndPoint' claims it is defined assembly `System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089', but it could not be found
If I open up the assembly browser in Xamarin, it appears the System.dll that's referenced does indeed have the Endpoint class defined.

One data point, which I'm not sure is relevant: the Pusher.Connections.Net library is dependent upon the WebSocket4Net assembly. If I attempt to add that assembly via the package manager, I get the following error:
Could not install package 'WebSocket4Net 0.8'. You are trying to install this package into a project that targets 'portable-net45+win+wp80+MonoTouch10+MonoAndroid10+xamarinmac20+xamarinios10', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author.
If however, I just go to the site, and download the assembly, and add a reference to it, it seems to satisfy all the WebSocket specific dependencies.
What am I missing? Thanks very much in advance.