Move StopwatchImplementation from coreimpl to core as _StopwatchImpl.

lib/core/stopwatch.dart

Index: lib/core/stopwatch.dart
diff --git a/lib/core/stopwatch.dart b/lib/core/stopwatch.dart
index e43f52d4bac7ced5274815a6b56b7d3910e81456..62bba16243d0c03462b17bc8647a180530dad476 100644
--- a/lib/core/stopwatch.dart
+++ b/lib/core/stopwatch.dart
@@ -14,7 +14,7 @@ abstract class Stopwatch {
    *
    *     Stopwatch stopwatch = new Stopwatch()..start();
    */
-  factory Stopwatch() => new StopwatchImplementation();
+  factory Stopwatch() => new _StopwatchImpl();
 
   /**
    * Starts the [Stopwatch]. The [elapsed] count is increasing monotonically.
@@ -63,3 +63,69 @@ abstract class Stopwatch {
   int frequency();
 
 }
+
+class _StopwatchImpl 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,

[floitsch, 2012/10/24 11:03:30]

has not been started yet, or is running. (NYC)

[Anders Johnsen, 2012/10/24 12:19:55]

No?:

  93   void stop() {
  94     if (_start === null || _stop !== null) {
  95       return;
  96     }
  97     _stop = _now();
  98   }

[Ivan Posva, 2012/10/26 02:52:26]

Florian, care to explain this?

[floitsch, 2012/10/26 09:25:15]

It was just a different way of saying the same thing. But we will keep it as is.

+  // or is running.
+  int _start;
+  int _stop;
+
+  _StopwatchImpl() : _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.

[Ivan Posva, 2012/10/26 02:52:26]

// may be 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();
+}