Add missing ResolveType call (fix issue 15738).

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 31335)
+++ runtime/vm/object.cc	(working copy)
@@ -1985,6 +1985,7 @@
       break;
     }
     sup_type = cls.super_type();
+    ClassFinalizer::ResolveType(cls, sup_type);
     cls = sup_type.type_class();
   } while (true);
   set_num_type_arguments(num_type_args);