Create read only handles for empty_array and sentinel objects

vm/exceptions.cc

Index: vm/exceptions.cc
===================================================================
--- vm/exceptions.cc	(revision 16331)
+++ vm/exceptions.cc	(working copy)
@@ -143,8 +143,8 @@
                                  const Instance& existing_stacktrace) {
   Instance& exception = Instance::Handle(incoming_exception.raw());
   if (exception.IsNull()) {
-    const Array& arguments = Array::Handle(Object::empty_array());
-    exception ^= Exceptions::Create(Exceptions::kNullThrown, arguments);
+    exception ^= Exceptions::Create(Exceptions::kNullThrown,
+                                    Object::empty_array());
   }
   uword handler_pc = 0;
   uword handler_sp = 0;