Update SDK with VM native extension development files.

Update SDK with VM native extension development files.

- dart_api.h
- import library on Windows
- dart_debugger_api.h

The debugger api is not necessarily needed at this point but I see no
reason to leave it out.

Dan, do I need to do something more than this to make sure that the
import library that is only built on Windows ends up in the final
SDK that we ship?

R=dgrove@google.com,whesse@google.com,iposva@google.com

Committed: https://code.google.com/p/dart/source/detail?r=7182

[Bill Hesse, 2012-05-01 08:07:27]

Looks correct to me.  I will let Dan approve it, though.

[Ivan Posva, 2012-05-01 08:51:00]

LGTM -ip

[dgrove, 2012-05-01 09:14:09]

lgtm