Internally measure isolate uptime with monotonic time.

runtime/vm/isolate.h

 // Copyright (c) 2013, 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.
 
 #ifndef RUNTIME_VM_ISOLATE_H_
 #define RUNTIME_VM_ISOLATE_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/atomic.h"
@@ skipping 186 matching lines @@
   void set_message_notify_callback(Dart_MessageNotifyCallback value) {
     message_notify_callback_ = value;
   }
 
   Thread* mutator_thread() const;
 
   const char* name() const { return name_; }
   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) {
     ASSERT(main_port_ == 0);  // Only set main port once.
     main_port_ = port;
   }
   Dart_Port origin_id() const { return origin_id_; }
   void set_origin_id(Dart_Port id) {
     ASSERT((id == main_port_ && origin_id_ == 0) || (origin_id_ == main_port_));
     origin_id_ = id;
@@ skipping 492 matching lines @@
   RawCode* ic_miss_code_;
   ObjectStore* object_store_;
   ClassTable class_table_;
   bool single_step_;
 
   ThreadRegistry* thread_registry_;
   SafepointHandler* safepoint_handler_;
   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_;
   uint64_t terminate_capability_;
   bool errors_fatal_;
   void* init_callback_data_;
   Dart_EnvironmentCallback environment_callback_;
   Dart_LibraryTagHandler library_tag_handler_;
   ApiState* api_state_;
   Debugger* debugger_;
@@ skipping 262 matching lines @@
   intptr_t* spawn_count_;
 
   Dart_IsolateFlags isolate_flags_;
   bool paused_;
   bool errors_are_fatal_;
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_ISOLATE_H_