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

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
===================================================================
--- runtime/vm/isolate.cc	(revision 40496)
+++ runtime/vm/isolate.cc	(working copy)
@@ -440,6 +440,7 @@
       thread_state_(NULL),
       tag_table_(GrowableObjectArray::null()),
       current_tag_(UserTag::null()),
+      default_tag_(UserTag::null()),
       metrics_list_head_(NULL),
       next_(NULL),
       REUSABLE_HANDLE_LIST(REUSABLE_HANDLE_INITIALIZERS)
@@ -498,6 +499,7 @@
       thread_state_(NULL),
       tag_table_(GrowableObjectArray::null()),
       current_tag_(UserTag::null()),
+      default_tag_(UserTag::null()),
       metrics_list_head_(NULL),
       next_(NULL),
       REUSABLE_HANDLE_LIST(REUSABLE_HANDLE_INITIALIZERS)
@@ -1124,6 +1126,9 @@
   // Visit the current tag which is stored in the isolate.
   visitor->VisitPointer(reinterpret_cast<RawObject**>(&current_tag_));
 
+  // Visit the default tag which is stored in the isolate.
+  visitor->VisitPointer(reinterpret_cast<RawObject**>(&default_tag_));
+
   // Visit the tag table which is stored in the isolate.
   visitor->VisitPointer(reinterpret_cast<RawObject**>(&tag_table_));
 
@@ -1312,12 +1317,18 @@
 
 
 void Isolate::set_current_tag(const UserTag& tag) {
-  intptr_t user_tag = tag.tag();
-  set_user_tag(static_cast<uword>(user_tag));
+  uword user_tag = tag.tag();
+  ASSERT(user_tag < kUwordMax);
+  set_user_tag(user_tag);
   current_tag_ = tag.raw();
 }
 
 
+void Isolate::set_default_tag(const UserTag& tag) {
+  default_tag_ = tag.raw();
+}
+
+
 void Isolate::VisitIsolates(IsolateVisitor* visitor) {
   if (visitor == NULL) {
     return;