Don't visit a frame's code object twice.

runtime/vm/stack_frame.cc

Index: runtime/vm/stack_frame.cc
diff --git a/runtime/vm/stack_frame.cc b/runtime/vm/stack_frame.cc
index a1fd87eda4606ed0f220501843ef9e7f2a833999..268dcb1737bcee746193d2cc33033a2d3d5f64a1 100644
--- a/runtime/vm/stack_frame.cc
+++ b/runtime/vm/stack_frame.cc
@@ -97,10 +97,6 @@ void StackFrame::VisitObjectPointers(ObjectPointerVisitor* visitor) {
   Code code;
   code = GetCodeObject();
   if (!code.IsNull()) {
-    // Visit the code object.
-    RawObject* raw_code = code.raw();
-    visitor->VisitPointer(&raw_code);
-
     // Optimized frames have a stack map. We need to visit the frame based
     // on the stack map.
     Array maps;