Update stopwatch to use patching to share sources instead of exposing

runtime/lib/clock.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.
-
-/**
- * The class [Clock] provides access to a monotonically incrementing clock
- * device.
- */
-class Clock {
-
-  /**
-   * Returns the current clock tick.
-   */
-  static int now() native "Clock_now";
-
-  /**
-   * Returns the frequency of clock ticks in Hz.
-   */
-  static int frequency() native "Clock_frequency";
-
-}