Use a monotonic clock in the implementation of Stopwatch.

runtime/lib/timeline.cc

 // Copyright (c) 2015, 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 "include/dart_api.h"
 
 #include "vm/native_entry.h"
 #include "vm/object.h"
@@ skipping 14 matching lines @@
 DEFINE_NATIVE_ENTRY(Timeline_getNextAsyncId, 0) {
   TimelineEventRecorder* recorder = Timeline::recorder();
   if (recorder == NULL) {
     return Integer::New(0);
   }
   return Integer::New(recorder->GetNextAsyncId());
 }
 
 
 DEFINE_NATIVE_ENTRY(Timeline_getTraceClock, 0) {
-  return Integer::New(OS::GetCurrentTraceMicros(), Heap::kNew, true);
+  return Integer::New(OS::GetCurrentMonotonicMicros(), Heap::kNew, true);
 }
 
 
 DEFINE_NATIVE_ENTRY(Timeline_reportTaskEvent, 6) {
   GET_NON_NULL_NATIVE_ARGUMENT(Integer, start, arguments->NativeArgAt(0));
   GET_NON_NULL_NATIVE_ARGUMENT(Integer, id, arguments->NativeArgAt(1));
   GET_NON_NULL_NATIVE_ARGUMENT(String, phase, arguments->NativeArgAt(2));
   GET_NON_NULL_NATIVE_ARGUMENT(String, category, arguments->NativeArgAt(3));
   GET_NON_NULL_NATIVE_ARGUMENT(String, name, arguments->NativeArgAt(4));
   GET_NON_NULL_NATIVE_ARGUMENT(String, args, arguments->NativeArgAt(5));
@@ skipping 122 matching lines @@
     return Object::null();
   }
   // json was allocated in the zone and a copy will be stored in event.
   event->SerializedJSON(json);
   event->Complete();
 
   return Object::null();
 }
 
 }  // namespace dart