Stopwatch.elapsed must always increase after start has been called.

dart/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.
  */
 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 _stop is null, then the [Stopwatch] has not been stopped yet,
+  // or is running.
   int _start;
   int _stop;
 
   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);
+      _stop = null;
     }
   }
 
   void stop() {
-    if (_start === null) {
+    if (_start === null || _stop !== 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();
@@ skipping 17 matching lines @@
 
   int elapsedInMs() {
     return (elapsed() * 1000) ~/ frequency();
   }
 
   int frequency() {
     return Clock.frequency();
   }
 
 }