[Unit tests] Mocking D objects

[Andrey via Digitalmars-d-learn]

Hello,
I know that D has build-in unit tests. If so, what mechanism D 
provides for mocking objects?
For example:
struct WebParser
{
   // ...
   int download(string path)
   {
       SomeHttpClient client(path);
       auto result = client.request(path, 10, "Qwerty");

       // ...

       return result.getSomething();
   }
}

Here I want to replace struct/class SomeHttpClient from 3d-party 
library with my own test implementation. Something like this 
maybe:
unittest
{
    SomeMagicMockMechanism!(SomeHttpClient, MyMockedClient);

    WebParser parser;
    auto value = parser.download("www.example.com"); // uses 
MyMockedClient.request
    assert(value == 10);

}