Avoid reading RawObject::class_ directly in the runtime.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 5420ed216d51fde98ca437a56c2ecc7a612c32e8..3eb9c37e48a7012caddc60ae57de4c8087152277 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -278,7 +278,9 @@ void Object::InitOnce() {
 
     Class fake;
     // Initialization from Class::New<Class>.
-    cls = class_class_;
+    // Directly set raw_ to break a circular dependency: SetRaw will attempt
+    // to lookup class class in the class table where it is not registered yet.
+    cls.raw_ = class_class_;
     cls.set_handle_vtable(fake.vtable());
     cls.set_instance_size(Class::InstanceSize());
     cls.set_next_field_offset(Class::InstanceSize());