Hide Isolate pointer from embedder

runtime/lib/isolate.cc

Index: runtime/lib/isolate.cc
diff --git a/runtime/lib/isolate.cc b/runtime/lib/isolate.cc
index 1f72dc2699851bb269f7e4d1f7dca9d05fca2c0f..12f834c4198cb5175aebd6878468dcd8f1df50e9 100644
--- a/runtime/lib/isolate.cc
+++ b/runtime/lib/isolate.cc
@@ -172,12 +172,11 @@ static bool CreateIsolate(Isolate* parent_isolate,
   }
 
   void* init_data = parent_isolate->init_callback_data();
-  Isolate* child_isolate = reinterpret_cast<Isolate*>(
-      (callback)(state->script_url(),
-                 state->function_name(),
-                 state->package_root(),
-                 init_data,
-                 error));
+  Isolate* child_isolate = Api::CastIsolate((callback)(state->script_url(),
+                                                       state->function_name(),
+                                                       state->package_root(),
+                                                       init_data,
+                                                       error));
   if (child_isolate == NULL) {
     return false;
   }