Restore back pointer from Instructions to Code

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index 925f56d4abf1f6a051d128f1a7ee14beb3a5e02e..b6f14f0b96ec6a593737eaeb0ace3019f9913596 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -1084,6 +1084,14 @@ class RawObjectPool : public RawObject {
 class RawInstructions : public RawObject {
   RAW_HEAP_OBJECT_IMPLEMENTATION(Instructions);
 
+  RawObject** from() {
+    return reinterpret_cast<RawObject**>(&ptr()->code_);
+  }
+  RawCode* code_;
+  RawObject** to() {
+    return reinterpret_cast<RawObject**>(&ptr()->code_);
+  }
+
   int32_t size_;
 
   // Variable length data follows here.