Internally measure isolate uptime with monotonic time.

runtime/vm/dart.h

 // 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.
 
 #ifndef RUNTIME_VM_DART_H_
 #define RUNTIME_VM_DART_H_
 
 #include "include/dart_api.h"
 #include "vm/allocation.h"
 #include "vm/snapshot.h"
@@ skipping 38 matching lines @@
                                      intptr_t snapshot_length,
                                      kernel::Program* kernel_program,
                                      void* data);
   static void RunShutdownCallback();
   static void ShutdownIsolate(Isolate* isolate);
   static void ShutdownIsolate();
 
   static Isolate* vm_isolate() { return vm_isolate_; }
   static ThreadPool* thread_pool() { return thread_pool_; }
 
-  // Returns a timestamp for use in debugging output in milliseconds
-  // since start time.
-  static int64_t timestamp();
+  static int64_t UptimeMicros();
+  static int64_t UptimeMillis() {
+    return UptimeMicros() / kMicrosecondsPerMillisecond;
+  }
 
   static void set_pprof_symbol_generator(DebugInfo* value) {
     pprof_symbol_generator_ = value;
   }
   static DebugInfo* pprof_symbol_generator() { return pprof_symbol_generator_; }
 
   static LocalHandle* AllocateReadOnlyApiHandle();
   static bool IsReadOnlyApiHandle(Dart_Handle handle);
 
   static uword AllocateReadOnlyHandle();
@@ skipping 47 matching lines @@
   }
   static Dart_EntropySource entropy_source_callback() {
     return entropy_source_callback_;
   }
 
  private:
   static void WaitForIsolateShutdown();
   static void WaitForApplicationIsolateShutdown();
 
   static Isolate* vm_isolate_;
-  static int64_t start_time_;
+  static int64_t start_time_micros_;
   static ThreadPool* thread_pool_;
   static DebugInfo* pprof_symbol_generator_;
   static ReadOnlyHandles* predefined_handles_;
   static Snapshot::Kind snapshot_kind_;
   static const uint8_t* instructions_snapshot_buffer_;
   static const uint8_t* data_snapshot_buffer_;
   static Dart_ThreadExitCallback thread_exit_callback_;
   static Dart_FileOpenCallback file_open_callback_;
   static Dart_FileReadCallback file_read_callback_;
   static Dart_FileWriteCallback file_write_callback_;
   static Dart_FileCloseCallback file_close_callback_;
   static Dart_EntropySource entropy_source_callback_;
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_DART_H_