Merge more Kernel infrastructure from kernel_sdk SDK fork.

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";
   UserTag makeCurrent() native "UserTag_makeCurrent";
 }
 
 @patch UserTag getCurrentTag() => _getCurrentTag();
 UserTag _getCurrentTag() native "Profiler_getCurrentTag";
 
 UserTag _getDefaultTag() native "UserTag_defaultTag";