Rename StopWatch to Stopwatch. Add new Stopwatch.start() and Stopwatch.reset().

corelib/src/implementation/stopwatch_implementation.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.
 
 /**
- * A simple implementation of the [StopWatch] interface.
+ * A simple implementation of the [Stopwatch] interface.
  */
-class StopWatchImplementation implements StopWatch {
+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.
+  // If _start is null, then the [Stopwatch] has not been started yet.
+  // If _stop is null, then the [Stopwatch] has not been stopped yet.
   int _start;
   int _stop;
 
-  StopWatchImplementation() : _start = null, _stop = null {}
+  StopwatchImplementation() : _start = null, _stop = null {}
+  StopwatchImplementation.start() : _start = null, _stop = null {
+    start();
+  }
 
   void start() {
     if (_start === null) {
       // This stopwatch has never been started.
       _start = Clock.now();
     } else {
       if (_stop === null) {
         return;
       }
       // Restarting this stopwatch. Prepend the elapsed time to the current
       // start time.
       _start = Clock.now() - (_stop - _start);
     }
   }
 
   void stop() {
     if (_start === null) {
       return;
     }
     _stop = Clock.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 = Clock.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) ? (Clock.now() - _start) : (_stop - _start);
   }
 
   int elapsedInUs() {
     return (elapsed() * 1000000) ~/ frequency();
   }
 
   int elapsedInMs() {
     return (elapsed() * 1000) ~/ frequency();
   }
 
   int frequency() {
     return Clock.frequency();
   }
 
 }