DependencyServiceStub.cs

public class DependencyServiceStub : IDependencyService
{
    readonly Dictionary<Type, object> registeredServices = new Dictionary<Type, object>();

    public void Register<T>(object impl)
    {
        registeredServices[typeof(T)] = impl;
    }

    public T Get<T>() where T : class
    {
        var service = (T)registeredServices[typeof(T)];

        return service;
    }
}