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 2998fd8b59fc84846f4d2009b8cde228b416ff66..ff66876b30f72dd48c17c2fe2e7c98393016f69e 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -470,6 +470,13 @@ 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) {               \
@@ -553,6 +560,7 @@ class Isolate : public BaseIsolate {
   InterruptableThreadState* thread_state_;
 
   VMTagCounters vm_tag_counters_;
+  uword user_tag_;
 
   // Isolate list next pointer.
   Isolate* next_;