member references to duck casts not converted to invoke runtimeservices

(excuse the title, i haven't tracked down the exact nature of this bug)

An error in svn that wasn't in 0.6. It's the cast(duck,A) line in the below sample (A as duck gives same error):

The error is essentially "NotImplemented: boo.lang.compiler.typesystem.builtinfunction" called from emitassembly.onmemberreferenceexpression

// cast(duck,A).Instance.doit()
is being converted in expandducktypedexpressions step to:
//Boo.Lang.Runtime.RuntimeServices.Invoke(cast(System.Object, A).Instance, 'doit', (,))

//begin example:
class A:
static public Instance as B = B()
class B:
def doit():
return 3
print cast(duck,A).Instance.doit()