Add a hash map for a raw object to id mapping, this table will be used during snapshot writing in o…

runtime/vm/heap.cc

Index: runtime/vm/heap.cc
diff --git a/runtime/vm/heap.cc b/runtime/vm/heap.cc
index 670c86844caed7e4c7c0b5183a84b2d33539387b..95fb0ae3515d27fc726e715990f16d451a5095fc 100644
--- a/runtime/vm/heap.cc
+++ b/runtime/vm/heap.cc
@@ -652,6 +652,18 @@ int64_t Heap::HashCount() const {
 }
 
 
+int64_t Heap::ObjectIdCount() const {
+  return new_weak_tables_[kObjectIds]->count() +
+      old_weak_tables_[kObjectIds]->count();
+}
+
+
+void Heap::ResetObjectIdTable() {
+  new_weak_tables_[kObjectIds]->Reset();
+  old_weak_tables_[kObjectIds]->Reset();
+}
+
+
 intptr_t Heap::GetWeakEntry(RawObject* raw_obj, WeakSelector sel) const {
   if (raw_obj->IsNewObject()) {
     return new_weak_tables_[sel]->GetValue(raw_obj);