Refactor 'dart:profiler' UserTag API

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);
   }
+  /* patch */ static UserTag get defaultTag => _getDefaultTag();
 }
 
 
 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";
+UserTag _getDefaultTag() native "UserTag_defaultTag";