Initial UserTag and dart:profiler library

runtime/vm/tags.cc

Index: runtime/vm/tags.cc
diff --git a/runtime/vm/tags.cc b/runtime/vm/tags.cc
index f05aa22cc77cf3b478de034d91fd3d78e946f146..403bce842cdb278db95a22f84cea02424de985df 100644
--- a/runtime/vm/tags.cc
+++ b/runtime/vm/tags.cc
@@ -6,6 +6,8 @@
 
 #include "vm/isolate.h"
 #include "vm/json_stream.h"
+#include "vm/object.h"
+#include "vm/object_store.h"
 
 namespace dart {
 
@@ -78,4 +80,17 @@ void VMTagCounters::PrintToJSONObject(JSONObject* obj) {
   }
 }
 
+
+const char* UserTags::TagName(uword tag_address) {
+  ASSERT(tag_address >= kUserTagIdOffset);
+  ASSERT(tag_address < kUserTagIdOffset + kMaxUserTags);
+  Isolate* isolate = Isolate::Current();
+  const UserTag& tag =
+      UserTag::Handle(isolate, UserTag::FindTagById(tag_address));
+  ASSERT(!tag.IsNull());
+  const String& label = String::Handle(isolate, tag.label());
+  return label.ToCString();
+}
+
+
 }  // namespace dart