Internally measure isolate uptime with monotonic time.

runtime/vm/heap.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/heap.h"
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/flags.h"
 #include "vm/isolate.h"
@@ skipping 743 matching lines @@
     "%" Pd ", %" Pd ", "  // new gen: capacity before/after
     "%" Pd ", %" Pd ", "  // new gen: external before/after
     "%" Pd ", %" Pd ", "  // old gen: in use before/after
     "%" Pd ", %" Pd ", "  // old gen: capacity before/after
     "%" Pd ", %" Pd ", "  // old gen: external before/after
     "%.3f, %.3f, %.3f, %.3f, "  // times
     "%" Pd ", %" Pd ", %" Pd ", %" Pd ", "  // data
     "]\n",  // End with a comma to make it easier to import in spreadsheets.
     isolate()->main_port(), space_str, GCReasonToString(stats_.reason_),
     stats_.num_,
-    MicrosecondsToSeconds(stats_.before_.micros_ - isolate()->start_time()),
+    MicrosecondsToSeconds(isolate()->UptimeMicros()),
     MicrosecondsToMilliseconds(stats_.after_.micros_ -
                                     stats_.before_.micros_),
     RoundWordsToKB(stats_.before_.new_.used_in_words),
     RoundWordsToKB(stats_.after_.new_.used_in_words),
     RoundWordsToKB(stats_.before_.new_.capacity_in_words),
     RoundWordsToKB(stats_.after_.new_.capacity_in_words),
     RoundWordsToKB(stats_.before_.new_.external_in_words),
     RoundWordsToKB(stats_.after_.new_.external_in_words),
     RoundWordsToKB(stats_.before_.old_.used_in_words),
     RoundWordsToKB(stats_.after_.old_.used_in_words),
@@ skipping 68 matching lines @@
   Dart::vm_isolate()->heap()->WriteProtect(false);
 }
 
 
 WritableVMIsolateScope::~WritableVMIsolateScope() {
   ASSERT(Dart::vm_isolate()->heap()->UsedInWords(Heap::kNew) == 0);
   Dart::vm_isolate()->heap()->WriteProtect(true);
 }
 
 }  // namespace dart