The forward declaration of Dart_SetReturnValue in native_arguments.h

The forward declaration of Dart_SetReturnValue in native_arguments.h
was causing the function to appear to be ambiguous on windows due to
the DART_EXTERN_C annotation.  Refactor the code so that a different
function is forward-declared in order to fix the build.

Another fix would have been to prefix all calls to Dart_SetReturnValue
by :: in lib/mirrors.cc, but that looks ugly.
Committed: https://code.google.com/p/dart/source/detail?r=11719

[turnidge, 2012-08-31 20:12:13]

TBR=iposva

I'm going to commit now to unwedge the build.  If you have comments, I will do a
follow-up CL.