Stop using void* types in the dart embedding api.

runtime/vm/port_test.cc

Index: runtime/vm/port_test.cc
===================================================================
--- runtime/vm/port_test.cc	(revision 1987)
+++ runtime/vm/port_test.cc	(working copy)
@@ -183,14 +183,14 @@
 // End-of-test marker.
 static const intptr_t kEOT = 0xFFFF;
 
-void* AllocIntData(intptr_t payload) {
+Dart_Message AllocIntData(intptr_t payload) {
   intptr_t* result = reinterpret_cast<intptr_t*>(malloc(sizeof(payload)));
   *result = payload;
-  return result;
+  return reinterpret_cast<Dart_Message>(result);
 }
 
 
-intptr_t GetIntData(void* data) {
+intptr_t GetIntData(Dart_Message data) {
   return *reinterpret_cast<intptr_t*>(data);
 }