Make Dart_IdentityEqual live up to its spec and not leak boxing implementation detail.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 56ce80753c1cc42c569ded00cc03d7d954f6945e..a9cb099f5ed885f88fd4fd93172d8b5c2500b5d7 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -3739,6 +3739,10 @@ class Instance : public Object {
                     const AbstractTypeArguments& type_instantiator,
                     Error* bound_error) const;
 
+  // Check whether this instance is identical to the argument according to the
+  // specification of dare:core's identical().
+  bool IsIdenticalTo(const Instance& other) const;
+
   bool IsValidNativeIndex(int index) const {
     return ((index >= 0) && (index < clazz()->ptr()->num_native_fields_));
   }