Hide Isolate pointer from embedder

runtime/vm/benchmark_test.h

Index: runtime/vm/benchmark_test.h
diff --git a/runtime/vm/benchmark_test.h b/runtime/vm/benchmark_test.h
index b26a13985aa74ea2f3085868eefe01b56f5c49c6..fd5c39aab09457a7651a3c6de4693f937bff379f 100644
--- a/runtime/vm/benchmark_test.h
+++ b/runtime/vm/benchmark_test.h
@@ -8,6 +8,7 @@
 #include "include/dart_api.h"
 
 #include "vm/dart.h"
+#include "vm/dart_api_impl.h"
 #include "vm/globals.h"
 #include "vm/heap.h"
 #include "vm/isolate.h"
@@ -77,12 +78,12 @@ class Benchmark {
   const char* score_kind() const { return score_kind_; }
   void set_score(int64_t value) { score_ = value; }
   int64_t score() const { return score_; }
-  Isolate* isolate() const { return reinterpret_cast<Isolate*>(isolate_); }
+  Isolate* isolate() const { return Api::CastIsolate(isolate_); }
 
   Dart_Isolate CreateIsolate(const uint8_t* buffer) {
     char* err = NULL;
     isolate_ = Dart_CreateIsolate(NULL, NULL, buffer, NULL, &err);
-    EXPECT(isolate_ != NULL);
+    EXPECT(isolate_ != DART_ILLEGAL_ISOLATE);
     free(err);
     return isolate_;
   }