Avoid reading RawObject::class_ directly in the runtime.

runtime/vm/heap.cc

Index: runtime/vm/heap.cc
diff --git a/runtime/vm/heap.cc b/runtime/vm/heap.cc
index 1a4df3c328eb4f54527af2524f8d398937bea608..5edb638e75a3e5654c503e48bf9dd1c0562d3d22 100644
--- a/runtime/vm/heap.cc
+++ b/runtime/vm/heap.cc
@@ -133,7 +133,7 @@ RawInstructions* Heap::FindObjectInCodeSpace(FindObjectVisitor* visitor) {
   // The code heap can only have RawInstructions objects.
   RawObject* raw_obj = code_space_->FindObject(visitor);
   ASSERT((raw_obj == Object::null()) ||
-         (raw_obj->ptr()->class_->ptr()->instance_kind_ == kInstructions));
+         (raw_obj->GetClassIndex() == kInstructions));
   return reinterpret_cast<RawInstructions*>(raw_obj);
 }