Refactor 'dart:profiler' UserTag API

runtime/vm/dart.cc

Index: runtime/vm/dart.cc
diff --git a/runtime/vm/dart.cc b/runtime/vm/dart.cc
index 5f88fbcd27b0c152ef2c3448efcc751e0938647f..66e9601944bcb92c6179fb135fdf8817ade2b68a 100644
--- a/runtime/vm/dart.cc
+++ b/runtime/vm/dart.cc
@@ -267,6 +267,10 @@ RawError* Dart::InitializeIsolate(const uint8_t* snapshot_buffer, void* data) {
   // Create tag table.
   isolate->set_tag_table(
       GrowableObjectArray::Handle(GrowableObjectArray::New()));
+  // Set up default UserTag.
+  const UserTag& default_tag = UserTag::Handle(UserTag::DefaultTag());
+  isolate->set_current_tag(default_tag);
+
   return Error::null();
 }