Initial UserTag and dart:profiler library

runtime/lib/profiler.dart

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "dart:_internal";
+
+patch class UserTag {
+  /* patch */ factory UserTag(String label) {
+    return new _UserTag(label);
+  }
+}
+
+
+class _UserTag implements UserTag {
+  factory _UserTag(String label) native "UserTag_new";
+  String get label native "UserTag_label";
+  void makeCurrent() native "UserTag_makeCurrent";
+}