Add support for deep stubbing

[BlackHC]

It would be nice to have the same that RETURNS_DEEP_STUBS in http://mockito.googlecode.com/svn/tags/1.8.3/javadoc/org/mockito/Mockito.html does.

[fibulwinter]

We can't create mock classes on the fly right now - so deep stubbing is impossible until we figure out how to do something like mock(Class). So it's blocked by https://github.com/fibulwinter/dart-mockito/issues/20

Also the original mockito feature has this warning:

This feature should rarely be required for regular clean code! Leave it for legacy code.

Do we really need it?

[matanlurey]

I think it's worth closing this :-/, considering it's basically deprecated in Java too.

/cc @TedSander

[TedSander]

Agreed