Dart_NewNativePort creates a port associated with a C handler
function. When messages come in on this port, they are forwarded to
the C function for processing.
To support this, refactored PortMap so that it operates on a new
MessageHandler type instead of directly on Isolates.
For now, native ports have a dedicated single thread. Eventually we
will back native ports (and possibly Isolates as well) by a shared
thread pool.
Review URL: https://chromiumcodereview.appspot.com//9169063
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@3804 260f80e4-7a28-3924-810f-c04153c831b5
turnidge@google.com