Use a monotonic clock in the implementation of Stopwatch.

runtime/vm/os_linux.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/globals.h"
 #if defined(TARGET_OS_LINUX)
 
 #include "vm/os.h"
 
 #include <errno.h>  // NOLINT
@@ skipping 378 matching lines @@
   // gettimeofday has microsecond resolution.
   struct timeval tv;
   if (gettimeofday(&tv, NULL) < 0) {
     UNREACHABLE();
     return 0;
   }
   return (static_cast<int64_t>(tv.tv_sec) * 1000000) + tv.tv_usec;
 }
 
 
-int64_t OS::GetCurrentTraceMicros() {
+int64_t OS::GetCurrentMonotonicMicros() {
   struct timespec ts;
   if (clock_gettime(CLOCK_MONOTONIC, &ts) != 0) {
     UNREACHABLE();
     return 0;
   }
   // Convert to microseconds.
   int64_t result = ts.tv_sec;
   result *= kMicrosecondsPerSecond;
   result += (ts.tv_nsec / kNanosecondsPerMicrosecond);
   return result;
@@ skipping 237 matching lines @@
 }
 
 
 void OS::Exit(int code) {
   exit(code);
 }
 
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_LINUX)