fix spelling in core libraries

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.
  */
 class Stopwatch {
   /**
    * Cached frequency of the system. Must be initialized in [_initTicker];
    */
   static int _frequency;
 
   // The _start and _stop fields capture the time when [start] and [stop]
   // are called respectively.
-  // If _stop is null, the stopwatched is running.
+  // If _stop is null, the stopwatch is running.
   int _start = 0;
   int _stop = 0;
 
   /**
    * Creates a [Stopwatch] in stopped state with a zero elapsed count.
    *
    * The following example shows how to start a [Stopwatch]
    * immediately after allocation.
    * ```
    * var stopwatch = new Stopwatch()..start();
@@ skipping 88 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();
 }