Use a monotonic clock in the implementation of Stopwatch.

runtime/lib/stopwatch.cc

 // Copyright (c) 2012, 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.
 
 #include "vm/bootstrap_natives.h"
 
 #include "vm/object.h"
 #include "vm/os.h"
 
 namespace dart {
 
 DEFINE_NATIVE_ENTRY(Stopwatch_now, 0) {
   // TODO(iposva): investigate other hi-res time sources such as cycle count.
-  return Integer::New(OS::GetCurrentTimeMicros());
+  return Integer::New(OS::GetCurrentMonotonicMicros());
 }
 
 
 DEFINE_NATIVE_ENTRY(Stopwatch_frequency, 0) {
   // TODO(iposva): investigate other hi-res time sources such as cycle count.
   return Integer::New(1000000);
 }
 
 }  // namespace dart