Hide Isolate pointer from embedder

runtime/bin/main.cc

Index: runtime/bin/main.cc
diff --git a/runtime/bin/main.cc b/runtime/bin/main.cc
index ef61bb46d712645e0929b8d9daf6de6a135bac9d..a708bf13c1ea1fc1fe72e3df36aa84d8a113e20a 100644
--- a/runtime/bin/main.cc
+++ b/runtime/bin/main.cc
@@ -592,8 +592,8 @@ static Dart_Isolate CreateIsolateAndSetupHelper(const char* script_uri,
                                isolate_data,
                                error);
 
-  if (isolate == NULL) {
-    return NULL;
+  if (isolate == ILLEGAL_ISOLATE) {
+    return ILLEGAL_ISOLATE;
   }
 
   Dart_EnterScope();
@@ -995,7 +995,7 @@ void main(int argc, char** argv) {
                                                      commandline_package_root,
                                                      &error,
                                                      &exit_code);
-  if (isolate == NULL) {
+  if (isolate == ILLEGAL_ISOLATE) {
     Log::PrintErr("%s\n", error);
     free(error);
     delete [] isolate_name;
@@ -1005,7 +1005,7 @@ void main(int argc, char** argv) {
 
   Dart_EnterIsolate(isolate);
   ASSERT(isolate == Dart_CurrentIsolate());
-  ASSERT(isolate != NULL);
+  ASSERT(isolate != ILLEGAL_ISOLATE);
   Dart_Handle result;
 
   Dart_EnterScope();