Optimized.js

Program.ReturnArgument = function (arg) {
    return arg;
};
Program.Main = function (args) {
    var a = new CustomType();
    a._ctor(1);
    var b = Program.ReturnArgument(a).MemberwiseClone();
    a.Value = 2;
    System.Console.WriteLine("a={0}, b={1}", a, b);
    b = Program.ReturnArgument(a).MemberwiseClone();
    System.Console.WriteLine("a={0}, b={1}", a, b);
    a.Value = 3;
    System.Console.WriteLine("a={0}, b={1}", a, b);
};

Unoptimized.js

Program.ReturnArgument = function (arg) {
    return arg;
};
Program.Main = function (args) {
    var a = new CustomType();
    a._ctor(1);
    var b = Program.ReturnArgument(a.MemberwiseClone()).MemberwiseClone();
    a.Value = 2;
    System.Console.WriteLine("a={0}, b={1}", a.MemberwiseClone(), b.MemberwiseClone());
    b = Program.ReturnArgument(a.MemberwiseClone()).MemberwiseClone();
    System.Console.WriteLine("a={0}, b={1}", a.MemberwiseClone(), b.MemberwiseClone());
    a.Value = 3;
    System.Console.WriteLine("a={0}, b={1}", a.MemberwiseClone(), b.MemberwiseClone());
};