Stop using void* types in the dart embedding api.

runtime/vm/dart_api_impl.h

Index: runtime/vm/dart_api_impl.h
===================================================================
--- runtime/vm/dart_api_impl.h	(revision 1987)
+++ runtime/vm/dart_api_impl.h	(working copy)
@@ -42,6 +42,12 @@
   static PersistentHandle* UnwrapAsPersistentHandle(const ApiState& state,
                                                     Dart_Handle object);
 
+  // Cast the internal Isolate* type to the external Dart_Isolate type.
+  static Dart_Isolate CastIsolate(Isolate* isolate);
+
+  // Cast a message byte array to the external Dart_Message type.
+  static Dart_Message CastMessage(uint8_t* message);
+
   // Gets the handle used to designate successful return.
   static Dart_Handle Success();