Basic tag infrastructure

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 5410ab82d7c7f5c87ac8161d5ea1f9d043913d18..4fea117524e373f158d5b3db5965b2a63aa065ac 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -14,6 +14,7 @@
 #include "vm/megamorphic_cache_table.h"
 #include "vm/random.h"
 #include "vm/store_buffer.h"
+#include "vm/tags.h"
 #include "vm/timer.h"
 
 namespace dart {
@@ -445,6 +446,14 @@ class Isolate : public BaseIsolate {
     return thread_state_;
   }
 
+  intptr_t vm_tag() const {
+    return vm_tag_;
+  }
+  void set_vm_tag(intptr_t tag) {
+    ASSERT(tag != VMTag::kInvalidTagId);
+    vm_tag_ = tag;
+  }
+
 #if defined(DEBUG)
 #define REUSABLE_HANDLE_SCOPE_ACCESSORS(object)                                \
   void set_reusable_##object##_handle_scope_active(bool value) {               \
@@ -523,6 +532,7 @@ class Isolate : public BaseIsolate {
   IsolateProfilerData* profiler_data_;
   Mutex profiler_data_mutex_;
   InterruptableThreadState* thread_state_;
+  intptr_t vm_tag_;
 
   // Isolate list next pointer.
   Isolate* next_;