1. Add user tag to the pointers traversed in the object store

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
===================================================================
--- runtime/vm/isolate.h	(revision 40496)
+++ runtime/vm/isolate.h	(working copy)
@@ -564,6 +564,9 @@
   static intptr_t current_tag_offset() {
     return OFFSET_OF(Isolate, current_tag_);
   }
+  static intptr_t default_tag_offset() {
+    return OFFSET_OF(Isolate, default_tag_);
+  }
 
 #define ISOLATE_METRIC_ACCESSOR(type, variable, name, unit)                    \
   type* Get##variable##Metric() { return &metric_##variable##_; }
@@ -578,6 +581,9 @@
   RawUserTag* current_tag() const { return current_tag_; }
   void set_current_tag(const UserTag& tag);
 
+  RawUserTag* default_tag() const { return default_tag_; }
+  void set_default_tag(const UserTag& tag);
+
   Metric* metrics_list_head() {
     return metrics_list_head_;
   }
@@ -691,6 +697,7 @@
   uword user_tag_;
   RawGrowableObjectArray* tag_table_;
   RawUserTag* current_tag_;
+  RawUserTag* default_tag_;
 
   Metric* metrics_list_head_;