Missing an assembly reference for System.dll in Unity project

Hi, I'm new to Unity and would like to debug some of the third party libraries that I use in my project. All these dlls use System.Diagnostics.Debug method for logging and don't print any logs on VS output console. I tried to override the logging mechanism by registering custom logger, but I got the following error:
The type or namespace name 'TraceListener' does not exist in the namespace 'System.Diagnostics' (are you missing an assembly reference?)

public class UnityLogger : System.Diagnostics.TraceListener
{
private static UnityLogger instance = new UnityLogger();
private UnityLogger()
{
System.Diagnostics.Debug.Listeners.Add(instance);
}
public static UnityLogger getInstance()
{
return instance;
}
public override void WriteLine(string message)
{
Debug.Log(message);
}
public override void Write(string message)
{
WriteLine(message);
}

Is it possible to use VS dependencies while building Unity project or is there any other way to fix it? It looks like System.dll for Unity is a bit different and doesn't contain some classes. I also tried to import System.dll for .NET manually, but it resulted error due to duplicated dependencies.

I'm using Unity 5.6.3 and VS 2015 update 3.