Revert "Switch libraries to using new tags."

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;
-
 /**
  * A simple [Stopwatch] interface to measure elapsed time.
  */
 abstract class Stopwatch {
   /**
    * Creates a [Stopwatch] in stopped state with a zero elapsed count.
    *
    * The following example shows how to start a [Stopwatch]
    * right after allocation.
    *
@@ skipping 106 matching lines @@
 
   int get elapsedMilliseconds {
     return (elapsedTicks * 1000) ~/ frequency;
   }
 
   int get frequency => _frequency();
 
   external static int _frequency();
   external static int _now();
 }