Make `Stopwatch` logic simpler.

sdk/lib/core/stopwatch.dart

Index: sdk/lib/core/stopwatch.dart
diff --git a/sdk/lib/core/stopwatch.dart b/sdk/lib/core/stopwatch.dart
index e923083a7d174b7d6e611bec9562be51f6579684..8049d99f40c68a9459738520ef81b92d32444140 100644
--- a/sdk/lib/core/stopwatch.dart
+++ b/sdk/lib/core/stopwatch.dart
@@ -9,31 +9,35 @@ part of dart.core;
  */
 class Stopwatch {
   /**
-   * Frequency of the elapsed counter in Hz.
+   * Cached frequency of the system. Must be initialized in [_initTicker];
    */
-  int get frequency => _frequency;
+  static int _frequency;

[Lasse Reichstein Nielsen, 2016/11/09 13:29:08]

Moved field to here, getter below constructor.

[floitsch, 2016/11/10 16:31:57]

Acknowledged.

 
   // 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;
+  // If _stop is null, the stopwatched is running.

[floitsch, 2016/11/10 16:31:57]

stopwatch

+  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.
-   *
-   *     Stopwatch stopwatch = new Stopwatch()..start();
+   * ```dart

[floitsch, 2016/11/10 16:31:57]

No need for "dart".
That should be the default in dartdocs.

[Lasse Reichstein Nielsen, 2016/11/11 09:00:55]

Done.

+   * Stopwatch stopwatch = new Stopwatch()..start();

[floitsch, 2016/11/10 16:31:57]

Maybe switch to 'var'.

[Lasse Reichstein Nielsen, 2016/11/11 09:00:55]

Done.

+   * ```
    */
   Stopwatch() {
-    _initTicker();
+    if (_frequency == null) _initTicker();
   }
 
   /**
+   * Frequency of the elapsed counter in Hz.
+   */
+  int get frequency => _frequency;
+
+  /**
    * Starts the [Stopwatch].
    *
    * The [elapsed] count is increasing monotonically. If the [Stopwatch] has
@@ -43,14 +47,10 @@ class Stopwatch {
    * If the [Stopwatch] is currently running, then calling start does nothing.
    */
   void start() {
-    if (isRunning) return;
-    if (_start == null) {
-      // This stopwatch has never been started.
-      _start = _now();
-    } else {
-      // Restart this stopwatch. Prepend the elapsed time to the current
+    if (_stop != null) {
+      // (Re)start this stopwatch. Prepend the elapsed time to the current

[floitsch, 2016/11/10 16:31:57]

Maybe?

(Re)start this stopwatch. Move the start-point forward by the gap since the
watch was stopped.

In the end it doesn't really matter, though.

[Lasse Reichstein Nielsen, 2016/11/11 09:00:55]

Reworeded to:
... Don't count the time while the stopwatch has been stopped.

       // start time.
-      _start = _now() - (_stop - _start);
+      _start += _now() - _stop;
       _stop = null;
     }
   }
@@ -63,8 +63,7 @@ class Stopwatch {
    * effect.
    */
   void stop() {
-    if (!isRunning) return;
-    _stop = _now();
+    _stop ??= _now();

[floitsch, 2016/11/10 16:31:57]

I found the `isRunning` more telling, but the file is small enough to see what
happens.

[Lasse Reichstein Nielsen, 2016/11/11 09:00:55]

It is more readable, but it's also calling an overridable member which is bad
style.
We could add a private _isRunning, but I don't think it's worth it.

The real problem is probably that you can extend Stopwatch, but that's a 2.0
change if we want to prohibit that.

   }
 
   /**
@@ -73,15 +72,7 @@ class Stopwatch {
    * This method does not stop or start the [Stopwatch].
    */
   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;
-    }
+    _start = _stop ?? _now();
   }
 
   /**
@@ -96,10 +87,7 @@ class Stopwatch {
    * The elapsed number of clock ticks increases by [frequency] every second.
    */
   int get elapsedTicks {
-    if (_start == null) {
-      return 0;
-    }
-    return (_stop == null) ? (_now() - _start) : (_stop - _start);
+    return (_stop ?? _now()) - _start;
   }
 
   /**
@@ -125,14 +113,9 @@ class Stopwatch {
 
 
   /**
-   * Returns wether the [StopWatch] is currently running.
+   * Returns whether the [StopWatch] is currently running.

[floitsch, 2016/11/10 16:31:57]

Remove `Returns`.

[Lasse Reichstein Nielsen, 2016/11/11 09:00:55]

Removed everywhere.

    */
-  bool get isRunning => _start != null && _stop == null;
-
-  /**
-   * Cached frequency of the system. Must be initialized in [_initTicker];
-   */
-  static int _frequency;
+  bool get isRunning => _stop == null;
 
   /**
    * Initializes the time-measuring system. *Must* initialize the [_frequency]