Move StopwatchImplementation from coreimpl to core as _StopwatchImpl.

lib/coreimpl/stopwatch_implementation.dart

-// Copyright (c) 2012, 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.
-
-/**
- * A simple implementation of the [Stopwatch] interface.
- */
-class StopwatchImplementation implements Stopwatch {
-  // The _start and _stop fields capture the time when [start] and [stop]
-  // are called respectively.
-  // If _start is null, then the [Stopwatch] has not been started yet.
-  // If _stop is null, then the [Stopwatch] has not been stopped yet,
-  // or is running.
-  int _start;
-  int _stop;
-
-  StopwatchImplementation() : _start = null, _stop = null {}
-
-  void start() {
-    if (_start === null) {
-      // This stopwatch has never been started.
-      _start = _now();
-    } else {
-      if (_stop === null) {
-        return;
-      }
-      // Restarting this stopwatch. Prepend the elapsed time to the current
-      // start time.
-      _start = _now() - (_stop - _start);
-      _stop = null;
-    }
-  }
-
-  void stop() {
-    if (_start === null || _stop !== null) {
-      return;
-    }
-    _stop = _now();
-  }
-
-  void reset() {
-    if (_start === null) return;
-    // If [_start] is not null, then the stopwatch had already been started. It
-    // may running right now.
-    _start = _now();
-    if (_stop !== null) {
-      // The watch is not running. So simply set the [_stop] to [_start] thus
-      // having an elapsed time of 0.
-      _stop = _start;
-    }
-  }
-
-  int elapsed() {
-    if (_start === null) {
-      return 0;
-    }
-    return (_stop === null) ? (_now() - _start) : (_stop - _start);
-  }
-
-  int elapsedInUs() {
-    return (elapsed() * 1000000) ~/ frequency();
-  }
-
-  int elapsedInMs() {
-    return (elapsed() * 1000) ~/ frequency();
-  }
-
-  int frequency() => _frequency();
-
-  external static int _frequency();
-  external static int _now();
-}