Internally measure isolate uptime with monotonic time.

runtime/vm/os_test.cc

Index: runtime/vm/os_test.cc
diff --git a/runtime/vm/os_test.cc b/runtime/vm/os_test.cc
index fe0160976331e7ad1f8912149220a4dd83fa6228..2ecc36b1bf929d7d9b219a36f17b864eaf194293 100644
--- a/runtime/vm/os_test.cc
+++ b/runtime/vm/os_test.cc
@@ -11,11 +11,12 @@
 namespace dart {
 
 UNIT_TEST_CASE(Sleep) {
-  int64_t start_time = OS::GetCurrentTimeMillis();
-  int64_t sleep_time = 702;
-  OS::Sleep(sleep_time);
-  int64_t delta = OS::GetCurrentTimeMillis() - start_time;
-  const int kAcceptableSleepWakeupJitter = 200;  // Measured in milliseconds.
+  // All times measured in microseconds.
+  int64_t start_time = OS::GetCurrentMonotonicMicros();
+  int64_t sleep_time = 702000;
+  OS::SleepMicros(sleep_time);
+  int64_t delta = OS::GetCurrentMonotonicMicros() - start_time;
+  const int kAcceptableSleepWakeupJitter = 200000;
   EXPECT_GE(delta, sleep_time - kAcceptableSleepWakeupJitter);
   EXPECT_LE(delta, sleep_time + kAcceptableSleepWakeupJitter);
 }