Internally measure isolate uptime with monotonic time.

runtime/vm/scavenger.cc

 // 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.
 
 #include "vm/scavenger.h"
 
 #include "vm/dart.h"
 #include "vm/dart_api_state.h"
 #include "vm/isolate.h"
 #include "vm/lockers.h"
@@ skipping 854 matching lines @@
     return;
   }
   Isolate* isolate = Isolate::Current();
   ASSERT(isolate != NULL);
   JSONObject space(object, "new");
   space.AddProperty("type", "HeapSpace");
   space.AddProperty("name", "new");
   space.AddProperty("vmName", "Scavenger");
   space.AddProperty("collections", collections());
   if (collections() > 0) {
-    int64_t run_time = OS::GetCurrentTimeMicros() - isolate->start_time();
+    int64_t run_time = isolate->UptimeMicros();
     run_time = Utils::Maximum(run_time, static_cast<int64_t>(0));
     double run_time_millis = MicrosecondsToMilliseconds(run_time);
     double avg_time_between_collections =
         run_time_millis / static_cast<double>(collections());
     space.AddProperty("avgCollectionPeriodMillis",
                       avg_time_between_collections);
   } else {
     space.AddProperty("avgCollectionPeriodMillis", 0.0);
   }
   space.AddProperty64("used", UsedInWords() * kWordSize);
@@ skipping 10 matching lines @@
 }
 
 
 void Scavenger::FreeExternal(intptr_t size) {
   ASSERT(size >= 0);
   external_size_ -= size;
   ASSERT(external_size_ >= 0);
 }
 
 }  // namespace dart