Never return a Dart_Handle on a dart::Class from the embedding API.

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index 32f5d83098f9f5e5f09850f2ab1686324d36630f..0e28b3190f01d18be0c3bbed79a8c91cbf64ee5a 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -347,6 +347,10 @@ class RawObject {
     ptr()->tags_ = RememberedBit::update(false, tags);
   }
 
+  bool IsRawClass() {
+    return IsHeapObject() && (GetClassId() == kClassCid);
+  }
+
   bool IsDartInstance() {
     return (!IsHeapObject() || (GetClassId() >= kInstanceCid));
   }