Initial UserTag and dart:profiler library

runtime/lib/profiler.dart

Index: runtime/lib/profiler.dart
diff --git a/runtime/lib/profiler.dart b/runtime/lib/profiler.dart
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+// 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";
+}
+
+patch UserTag getCurrentTag() => _getCurrentTag();
+UserTag _getCurrentTag() native "Profiler_getCurrentTag";
+
+patch UserTag clearCurrentTag() => _clearCurrentTag();
+UserTag _clearCurrentTag() native "Profiler_clearCurrentTag";