Internally measure isolate uptime with monotonic time.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 4eb574086c28bedc6edaf4f9baf228c226d5b4ba..88df144a37db1f63bf92d2df1e90c4a8e926525a 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -204,7 +204,7 @@ class Isolate : public BaseIsolate {
   const char* debugger_name() const { return debugger_name_; }
   void set_debugger_name(const char* name);
 
-  int64_t start_time() const { return start_time_; }
+  int64_t UptimeMicros() const;
 
   Dart_Port main_port() const { return main_port_; }
   void set_main_port(Dart_Port port) {
@@ -717,7 +717,7 @@ class Isolate : public BaseIsolate {
   Dart_MessageNotifyCallback message_notify_callback_;
   char* name_;
   char* debugger_name_;
-  int64_t start_time_;
+  int64_t start_time_micros_;
   Dart_Port main_port_;
   Dart_Port origin_id_;  // Isolates created by spawnFunc have some origin id.
   uint64_t pause_capability_;