Internally measure isolate uptime with monotonic time.

runtime/vm/service.cc

Index: runtime/vm/service.cc
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index 9ea1c512dfe2ebd1a30bfe9c065ebe9d468243ec..bdc71e989bc89aeac33119903309465443afa505 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -3785,7 +3785,6 @@ void Service::PrintJSONForVM(JSONStream* js, bool ref) {
   if (ref) {
     return;
   }
-  Isolate* vm_isolate = Dart::vm_isolate();
   jsobj.AddProperty("architectureBits", static_cast<intptr_t>(kBitsPerWord));
   jsobj.AddProperty("targetCPU", CPU::Id());
   jsobj.AddProperty("hostCPU", HostCPUFeatures::hardware());
@@ -3793,9 +3792,8 @@ void Service::PrintJSONForVM(JSONStream* js, bool ref) {
   jsobj.AddProperty("_profilerMode", FLAG_profile_vm ? "VM" : "Dart");
   jsobj.AddProperty64("pid", OS::ProcessId());
   jsobj.AddProperty64("_maxRSS", OS::MaxRSS());
-  int64_t start_time_millis =
-      (vm_isolate->start_time() / kMicrosecondsPerMillisecond);
-  jsobj.AddPropertyTimeMillis("startTime", start_time_millis);
+  jsobj.AddPropertyTimeMillis(
+      "startTime", OS::GetCurrentTimeMillis() - Dart::UptimeMillis());
   // Construct the isolate list.
   {
     JSONArray jsarr(&jsobj, "isolates");