Add Stopwatch.isRunning.

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 7d8ce334cd1d1c8a748af7295813cb22d1bf7776..b446069f70c1ad298a441051ec28816114d9149a 100644
--- a/sdk/lib/core/stopwatch.dart
+++ b/sdk/lib/core/stopwatch.dart
@@ -63,6 +63,11 @@ abstract class Stopwatch {
    * Returns the frequency of the elapsed counter in Hz.
    */
   int get frequency;
+
+  /**
+   * Returns wether the [StopWatch] is currently running.
+   */
+  bool get isRunning;
 }
 
 class _StopwatchImpl implements Stopwatch {
@@ -77,14 +82,12 @@ class _StopwatchImpl implements Stopwatch {
   _StopwatchImpl() : _start = null, _stop = null {}
 
   void start() {
+    if (isRunning) return;
     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
+      // Restart this stopwatch. Prepend the elapsed time to the current
       // start time.
       _start = _now() - (_stop - _start);
       _stop = null;
@@ -92,9 +95,7 @@ class _StopwatchImpl implements Stopwatch {
   }
 
   void stop() {
-    if (_start == null || _stop != null) {
-      return;
-    }
+    if (!isRunning) return;
     _stop = _now();
   }
 
@@ -127,6 +128,8 @@ class _StopwatchImpl implements Stopwatch {
 
   int get frequency => _frequency();
 
+  bool get isRunning => _start != null && _stop == null;
+
   external static int _frequency();
   external static int _now();
 }