Initial UserTag and dart:profiler library

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 7bb26a5cc192e5cb789c2f38387d56255af03f93..a2b0112a8a57acd292b1f73fb61369799d6732d0 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -451,6 +451,13 @@ class Isolate : public BaseIsolate {
     return &vm_tag_counters_;
   }
 
+  uword user_tag() const {
+    return user_tag_;
+  }
+  void set_user_tag(uword tag) {
+    user_tag_ = tag;
+  }
+
 #if defined(DEBUG)
 #define REUSABLE_HANDLE_SCOPE_ACCESSORS(object)                                \
   void set_reusable_##object##_handle_scope_active(bool value) {               \
@@ -532,6 +539,7 @@ class Isolate : public BaseIsolate {
   InterruptableThreadState* thread_state_;
 
   VMTagCounters vm_tag_counters_;
+  uword user_tag_;
 
   // Isolate list next pointer.
   Isolate* next_;