Revert "Remaining private libs"

sdk/lib/core/stopwatch.dart

 // Copyright (c) 2011, 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.
 
-part of "dart:core";
+part of "core.dart";
 
 /**
  * A simple stopwatch interface to measure elapsed time.
  */
 class Stopwatch {
   /**
    * Cached frequency of the system. Must be initialized in [_initTicker];
    */
   static int _frequency;
 
@@ skipping 100 matching lines @@
    */
   bool get isRunning => _stop == null;
 
   /**
    * Initializes the time-measuring system. *Must* initialize the [_frequency]
    * variable.
    */
   external static void _initTicker();
   external static int _now();
 }