Only call OS::GetCurrentThreadCPUMicros when a TimelineDurationScope is enabled

runtime/vm/timeline.cc

 // Copyright (c) 2015, 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 <cstdlib>
 
 #include "vm/atomic.h"
 #include "vm/isolate.h"
 #include "vm/json_stream.h"
 #include "vm/lockers.h"
@@ skipping 866 matching lines @@
   }
   event->StealArguments(arguments_length_, arguments_);
   arguments_length_ = 0;
   arguments_ = NULL;
 }
 
 
 TimelineDurationScope::TimelineDurationScope(TimelineStream* stream,
                                              const char* label)
     : TimelineEventScope(stream, label) {
-  if (!FLAG_support_timeline) {
+  if (!FLAG_support_timeline || !enabled()) {
     return;
   }
   timestamp_ = OS::GetCurrentMonotonicMicros();
   thread_timestamp_ = OS::GetCurrentThreadCPUMicros();
 }
 
 
 TimelineDurationScope::TimelineDurationScope(Thread* thread,
                                              TimelineStream* stream,
                                              const char* label)
     : TimelineEventScope(thread, stream, label) {
-  if (!FLAG_support_timeline) {
+  if (!FLAG_support_timeline || !enabled()) {
     return;
   }
   timestamp_ = OS::GetCurrentMonotonicMicros();
   thread_timestamp_ = OS::GetCurrentThreadCPUMicros();
 }
 
 
 TimelineDurationScope::~TimelineDurationScope() {
   if (!FLAG_support_timeline) {
     return;
@@ skipping 784 matching lines @@
 TimelineEventBlock* TimelineEventBlockIterator::Next() {
   ASSERT(current_ != NULL);
   TimelineEventBlock* r = current_;
   current_ = current_->next();
   return r;
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart