Create read only handles for empty_array and sentinel objects

vm/code_descriptors_test.cc

Index: vm/code_descriptors_test.cc
===================================================================
--- vm/code_descriptors_test.cc	(revision 16415)
+++ vm/code_descriptors_test.cc	(working copy)
@@ -277,9 +277,8 @@
   // Now invoke 'A.moo' and it will trigger a GC when the native function
   // is called, this should then cause the stack map of function 'A.foo'
   // to be traversed and the appropriate objects visited.
-  const Array& args = Array::Handle(Object::empty_array());
-  const Object& result = Object::Handle(DartEntry::InvokeStatic(function_foo,
-                                                                args));
+  const Object& result = Object::Handle(
+      DartEntry::InvokeStatic(function_foo, Object::empty_array()));
   EXPECT(!result.IsError());
 }